Collaborative planning and prediction: Does group discussion affect optimistic biases in time estimation?

Organizational Behavior and Human Decision Processes 97 (2005) 47–63

www.elsevier.com/locate/obhdp

Collaborative planning and prediction: Does group discussion aVect optimistic biases in time estimation? �

Roger Buehler a,¤, Deanna Messervey a, Dale GriYn b

a Psychology Department, Wilfrid Laurier University, Waterloo, Ont., Canada N2L 3C5b Faculty of Commerce and Business Administration, University of British Columbia, Canada

Received 10 November 2004Available online 1 April 2005

Abstract

Intuition and previous research indicate that individuals commonly display an optimistic bias in time prediction. The presentstudies extend research on task completion forecasts to examine tasks performed collaboratively by groups, and predictions gener-ated through group discussion. Participants predicted—individually and collaboratively—when they would complete upcominggroup projects ranging from brief laboratory tasks to extensive real-world projects, and their actual completion times were measured.Results supported the three guiding hypotheses. First, there was an optimistic bias for both individual (Studies 1 and 2) and grouppredictions (Studies 1–3). Second, predictions generated through group discussion were more optimistic than those generated indi-vidually. Third, this “group accentuation” eVect was mediated by the informational focus at the time of prediction. Group discussionheightened participants’ tendency to focus primarily on factors promoting successful task completion, and this selective focus on“planning for success” enhanced their optimistic outlook. Discussion centers on theoretical contributions to the individual andgroup decision making literatures, as well as applied implications for planning and forecasting within organizational contexts. 2005 Elsevier Inc. All rights reserved.

Keywords: Prediction; Planning; Forecasting; Optimistic bias; Group judgment; Time estimation

Introduction

Two facts about the process of planning and predic-tion are clear: First, both mundane, everyday projects(Buehler, GriYn, & Ross, 2002), and important, costly,and strategically vital projects (Flyvbjerg, Holme, &Soren, 2002; Hall, 1980) are typically subject to optimis-tic biases in planning. Second, most projects are plannedand carried out not by individuals, but by groups such ascommittees, teams, or work groups. Nonetheless, theoverwhelming majority of research on planning

� This research was supported by a grant to the Wrst author from theSocial Sciences and Humanities Research Council of Canada. Wethank Nicole Ethier, Elaine Restorick, Evelina Rog, and Vassili Spyro-poulos for their assistance with the research.

* Corresponding author. Fax: +1 519 746 7605.E-mail address: [email protected] (R. Buehler).

0749-5978/$ - see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.obhdp.2005.02.004

processes has examined the processes and outcomes ofde-contextualized individuals making predictions. Ouraim in the present research is to extend the existingscholarship on task completion forecasts to examinetasks carried out collaboratively by groups. In particular,we compare the outcome biases and planning processesassociated with predictions generated collaboratively,through group discussion, with those found in predic-tions generated by individual group members.

Individual prediction

Research on task completion forecasts has conWrmedan all too familiar observation from daily life: peopleoften predict they will Wnish their tasks sooner than theyactually do. This prediction bias has been documented inresearch examining the “planning fallacy” (Kahneman& Tversky, 1979), a speciWc form of optimistic bias in

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48 R. Buehler et al. / Organizational Behavior and Human Decision Processes 97 (2005) 47–63

which people make highly favorable estimates of thetime it will take to complete an upcoming task, eventhough they are fully aware that similar tasks have takenlonger in the past (for a review see Buehler et al., 2002).This optimistic bias is robust.1 It has been demonstratedfor a variety of tasks in both the laboratory and the realworld (Buehler, GriYn, & Ross, 1994; Byram, 1997; Tay-lor, Pham, Rivkin, & Armor, 1998); it generalizes acrossindividual diVerences in personality and culture (Buehler& GriYn, 2003; Buehler et al., 2002); and it has beenresistant to several “debiasing” interventions (Byram,1997; GriYn & Buehler, 1999; Newby-Clark, Ross,Buehler, Koehler, & GriYn, 2000).

The planning fallacy seems to have both cognitiveand motivational underpinnings. In their classic analysis,Kahneman and Tversky (1979) introduced a visual met-aphor to distinguish between two approaches to predic-tion. When taking an inside view, people focus narrowlyon case-speciWc features of the task at hand and con-struct a plan or scenario of how it will unfold. When tak-ing an outside view, people consider factors apart fromthe target task itself; they view the task in a broader con-text that includes relevant past experiences, the experi-ences of others, and background events that mayinXuence progress. According to this (individual) cogni-tive account, people’s overly optimistic predictions stemlargely from their tendency to adopt the inside approachat the expense of the outside view. People tend to focuson a case-speciWc plan or scenario for the upcoming taskand thus disregard other sources of information relevantto making accurate forecasts, including: their own pastexperiences (personal base rates), others’ experiences(population base rates), and possible obstacles. This cog-nitive process model has been supported by studies usingon-line verbal protocols (Buehler et al., 1994; Buehler,GriYn, & MacDonald, 1997; GriYn & Buehler, 1999)and thought-focus manipulations (Buehler & GriYn,2003).

Motivation also plays an important role in this bias,in part by further exacerbating the inside approach toprediction. Buehler et al. (1997) demonstrated the inter-play between motivation and cognition in producingoverly optimistic predictions. In a Weld study, taxpayerswho expected an income tax refund, and were thusstrongly motivated to Wle their tax return early, esti-mated they would Wle their return 10 days earlier onaverage than did taxpayers who did not expect a refund.In fact, the two groups did not diVer in terms of whenthey Wled their returns, which was later than eithergroup predicted. A follow-up experiment replicated the

1 In the present context, the term optimistic bias simply refers to atendency to predict that tasks will be Wnished sooner than they actuallyare. The term is not meant to imply that the bias has a motivational ba-sis or that it reXects dispositional optimism. Processes underlying thisoptimistic bias are discussed throughout the paper.

incentive eVect (such that incentives inXuenced predic-tions to a greater extent than actual performance; seealso Byram, 1997) and identiWed mediating cognitivemechanisms. Incentives for early task completionprompted increased attention to future plans andreduced attention to relevant past experiences—the verypattern of cognitive processes that fuels the planningfallacy.

Taken together, the cognitive and motivationalapproaches suggest one factor that should attenuate theplanning fallacy: people who are not personally involvedin a task (neutral observers) should be less prone tounrealistic predictions than those engaged in the task(actors). Observers typically do not have access to thewealth of information that actors possess about theirfuture plans and circumstances, making it diYcult forobservers to generate a detailed future scenario. Also,because neutral observers do not generally share thesame motivations as actors (e.g., to complete the taskpromptly), they may be less inclined to focus selectivelyon information that supports an optimistic forecast.Consistent with this reasoning, several studies have dem-onstrated that observer predictions are more realisticthan self predictions (Buehler et al., 1994; Epley & Dun-ning, 2000; Macdonald & Ross, 1999), in part, becauseobservers are more likely than actors to adopt an outsideapproach to prediction.

Collaborative prediction

Although researchers have shown great interest inpersonal planning and prediction, they have paid rela-tively little attention to the social-interactive context(Karniol & Ross, 1996; Ross & Buehler, 2001; Smith,1996). This neglect is surprising because planning andprediction is often a collaborative process. In organiza-tional settings, judgmental forecasting is often per-formed collaboratively (Armstrong, 1985); however,research on improving forecasting has focused largely onthe beneWts of aggregating independent forecasts (e.g.,Clemens & Winkler, 1993). Even when project managersrender a Wnal prediction alone, they usually interact withothers to collect information, opinions, and advice(Heath & Gonzalez, 1995). Similarly, people often con-sult family members or friends when predicting theirpersonal futures (Macdonald & Ross, 1999; Shrauger,Ram, Greninger, & Mariano, 1996). Whenever peopleundertake collaborative ventures, they need to synchro-nize their plans, schedules, and activities.

Our research on collaborative prediction is guided bythree speciWc questions: Are completion forecasts forgroup tasks optimistically biased? Do groups generatepredictions for group tasks that are more or less biasedthan those generated by individual group members?How does interaction and discussion inXuence thedegree of bias in predictions generated by groups?

R. Buehler et al. / Organizational Behavior and Human Decision Processes 97 (2005) 47–63 49

Optimistic bias for group projectsThere is some reason to expect that optimistic biases

found for individual tasks may not be found for group-based tasks. An important feature of group tasks is thatpeople are inter-dependent—they must often wait forcontributions of others before they can do their ownpart. In some ways, then, participants’ viewpoints aresimilar to that of an observer, which might give rise to anoutside perspective and mitigate the planning-basedoptimistic bias. However, membership in a collaborativegroup—even an ad hoc group such as those studiedhere—also creates important diVerences in perspectiverelative to neutral observers. Although a group membermay be partially an observer (for the other members’contributions) he or she is also an active participant (forhis or her own contribution) and has a stake in the taskas a whole. We reasoned that the role as an active con-tributor and stakeholder to a project would override anytendency to think as an observer—and thus we hypothe-sized that people’s predictions would be optimisticallybiased for group tasks. This hypothesis is also consistentwith a wealth of archival and anecdotal evidence thatlarge scale projects (which surely involve teams of plan-ners and consultants) routinely exceed deadlines andbudgets (Flyvbjerg et al., 2002; Hall, 1980).

EVect of group discussionIf group predictions are optimistically biased, should

we expect a smaller or larger error than is found forindividual predictions? Such comparisons are impor-tant because a common justiWcation for consulting withothers is to increase prediction accuracy. Perhaps sur-prisingly, then, a vast literature comparing the accuracyof group and individual judgment in many otherdomains has not yielded consistent evidence that inter-action with others improves the quality of judgment(for reviews see Gigone & Hastie, 1997; Kerr, Mac-Coun, & Kramer, 1996; Kerr & Tindale, 2004). Eventhe subset of studies most pertinent to ours—thoseexamining eVects of group discussion on judgmentalbias—reveals mixed evidence. Wright and colleagues(Wright & Wells, 1985; Wright, Luus, & Christie, 1990)have demonstrated that group discussion attenuatesseveral judgmental biases. For instance, groups did notexhibit the usual tendency to underutilize consensusinformation when making causal attributions. Analysesof verbal protocols revealed that only a minority ofindividuals appreciated the signiWcance of consensusinformation but, because this type of information wasso logically compelling, their arguments swayed theother group members.

Other research suggests that groups tend to accentu-ate rather than attenuate judgmental biases found inindividuals (Hinsz, Tindale, & Vollrath, 1997; Tindale,1993; Tindale, Smith, Thomas, Filkins, & SheVey, 1996).This “group accentuation eVect” appears for two judg-

mental biases—overconWdence and base-rate neglect –that are close cousins to the planning fallacy. First, theoverconWdence that people exhibit in the accuracy oftheir own judgment (Lichtenstein & FischhoV, 1977) isoften heightened by group discussion (Heath & Gonz-alez, 1995; Ono & Davis, 1988; Stephenson, Clark, &Wade, 1986; for exceptions to this pattern see Sniezek,1992). Second, interaction appears to exacerbate thejudgmental bias known as base-rate neglect (Kahneman& Tversky, 1973), whereby people focus disproportion-ately on individuating information and underweightgeneral base rates when making predictions and proba-bility estimates (Argote, Devadas, & Melone, 1990;Argote, Seabright, & Dyer, 1986; Tindale, 1993). Forexample, Argote et al. (1986) presented individuals andgroups with base-rate and individuating informationand asked them to estimate how likely it was that indi-viduals were members of professional categories. Judg-ments made by groups were less sensitive to the base-rateinformation than judgments made by individuals; inaddition, verbal protocols revealed even fewer mentionsof the base-rates in group discussions than in individualsthinking aloud. Given the conceptual similarity of theplanning fallacy to the phenomena of overconWdenceand base-rate neglect, we hypothesized that group dis-cussion will tend to heighten or accentuate the optimisticbias in people’s task completion forecasts.

Mediation of group discussion eVectsOur third question concerns the mechanism or pro-

cess by which collaboration may result in overly optimis-tic forecasts. In this regard, it is worth noting that thehypothesized “group accentuation” eVect resemblesgroup polarization eVects (Myers & Lamm, 1976)observed in the realm of attitudinal judgment. There areseveral processes by which these eVects may occur. First,persuasive arguments theory (Vinokur & Burnstein,1974) indicates that shifts in the direction of an initialbias or tendency occur because most of the argumentsand information raised during discussion will supportthat view (see also Hinsz et al., 1997; Tindale, 1993). Sec-ond, engaging in discussion may prompt people to con-struct a more coherent rationale for their own (usuallyoptimistic) initial views, and this process of rationaleconstruction serves to bolster their conWdence (Heath &Gonzalez, 1995). Third, normative inXuence occurs whenpeople want their expressed views to compare favorablywith those of other group members. In the presentdomain, where successful task completion is a sharedgoal, group members may exaggerate their optimism,and suppress pessimism, to demonstrate commitmentand loyalty to the project (Kahneman & Lovallo, 1993).In this way concerns for group approval may result inunrealistic views. Finally, an additional reason that opti-mism may be more pronounced in group contexts is thatindividual accountability for the estimate—as well as


any negative consequences for being unrealistic—may bediVused somewhat.

The purpose of the present research was not to disen-tangle these psychological processes. Indeed, these pro-cesses are largely complementary and produce similareVects; in each case group discussion prompts individu-als to focus selectively on information that supports oraccentuates the initial inclination of individual groupmembers. In the present judgment domain, we believethat people’s initial inclination is to focus narrowly onthoughts of successful task completion. We expect thatgroup discussion will reinforce this tendency, promptinggroup members to focus even more selectively on taskrelevant information (plans for task completion, quali-ties of the task, qualities of the group members, qualitiesof the situation) which suggests the task will be com-pleted promptly, and consequently to neglect or under-weight information suggesting a task may take longerthan desired.

The present research

The present research tested three hypotheses concern-ing task completion forecasts for group projects. First,there will be an optimistic bias in prediction, such thatpeople will expect their group projects to be Wnished, onaverage, earlier than they actually are. Second, there willbe a “group accentuation” eVect on the optimistic bias,such that predictions based on group discussion will bemore optimistic than those generated by individualgroup members. Third, group discussion will promptpeople to strengthen their selective focus on factors pro-moting successful task completion (e.g., qualities of thetask, the situation, and the group members which sug-gest a speedy completion time) and this selective focuswill mediate the group accentuation eVect on the opti-mistic bias.

The hypotheses were tested in three studies using aprospective design, in which participants predicted whena collaborative venture would be Wnished, and were fol-lowed up so that the actual completion time could beassessed. The target tasks included real-world projects,brief laboratory-based tasks, and take-home assign-ments. In each study, the predicted and actual comple-tion times were compared to determine whether peopleexhibited a systematic optimistic bias in their predic-tions. The studies included two commonly studied vari-ants of group prediction. In the consensus conditiongroups of participants arrived at a single prediction thatrepresented the consensus of the group. In the consulta-tion condition participants discussed their predictionswith fellow group members before rendering a Wnal pre-diction privately. Although we did not hypothesize spe-ciWc diVerences between these two conditions, we soughtto determine whether eVects of group discussion wouldgeneralize across both types of group-based prediction.

In addition, to explore processes underlying the eVects ofgroup discussion on prediction, Studies 2 and 3 includedmeasures that assessed the focus of participants’thoughts as they were generating their predictions.

Study 1

The primary purpose of the initial study was to testthe Wrst hypothesis—that predicted completion times forgroup-based projects tend to be optimistically biased—using a real-world task that was important to the partic-ipants. Undergraduate Business students predicted whenthey would complete each phase of a course project to becompleted in small groups, and later they reported whenthe work was actually Wnished. The study also provideda preliminary comparison of individual and collabora-tive predictions by using a simple pre-test/post-testdesign. Individual predictions were generated by eachgroup member, and then later each group arrived at asingle group prediction through consensus.

Method

ParticipantsParticipants in the initial session were 69 (34 male and

35 female) undergraduate students enrolled in a courseon organizational behavior. Sixty-seven of the partici-pants (33 male and 34 female) completed the second ses-sion, and received payment of $10.

ProcedureThe initial session was conducted during the Wrst

week of an organizational behavior course in which stu-dents were required to complete a group project worth25% of their grade. The project involved a written caseanalysis of a hypothetical business scenario, andincluded four sections: (a) statement of the problem, (b)analysis of the problem, (c) alternative solutions, and (d)recommended solution and implementation. The projectwas due nine weeks (63 days) after the initial session.After reviewing the project instructions, the instructordivided the class into 15 working groups of either four orWve students, allowed the group members to becomeacquainted, and then turned the class over to the experi-menter.

The experimenter invited students to participate in astudy examining how groups work together to completevarious projects, and all students agreed to participate.Participants Wrst completed an individual predictionquestionnaire that asked them to predict when (date andtime) the group would Wnish each section of the project(deWned as when the group would arrive at its “Wnalanswer” for the section and record it in writing). Partici-pants were instructed to make their predictions as accu-rately as possible without consulting group members,


and were provided with the aid of a calendar. As supple-mentary measures, participants also estimated howmany meetings their group would have outside of classtime, and rated the extent to which the task would beenjoyable, diYcult, and stressful on a scale ranging from1 (not at all) to 7 (extremely).

Next participants were asked to complete the groupprediction questionnaire, which contained the same ques-tions they had answered individually. Participants wereallowed 10 min to work through the questionnairetogether as a group, with the goal of making predictionsas accurately as possible. They were instructed to discusseach question and provide a single, consensus answerthat best represented the views of the group.

The second session was conducted during class time,three days after the project deadline. In individual ques-tionnaires, participants reported when (date and time)the group Wnished each section of the project. They alsoindicated how many meetings their group held outside ofclass time, and rated the extent to which the project wasenjoyable, diYcult, and stressful on a scale ranging from1 (not at all) to 7 (extremely).

Results and discussion

Each date/time variable was converted to a number ofdays (and fraction of days) from the initial research ses-sion. Because the responses of linked participants in thegroup conditions are not statistically independent, weaveraged across individual responses, and treated thegroup as the unit of analysis (Kenny, Kashy, & Bolger,1998).2 Means are presented in Table 1.

To test the Wrst hypothesis, we compared the pre-dicted completion times with actual completion times

2 For the type of data structure used in our primary analyses, the re-sults of these group-mean analyses are identical to those using nestedmodel analyses using individual scores (Raudenbush, 1993).

Table 1Predicted and actual completion times (in days) for the four projectcomponents as a function of group discussion (Study 1)

Project component

Individual prediction

Group prediction

Actual

Statement of problemM 31.07 22.16 53.27SD 9.99 8.72 3.04

Analysis of problemM 36.75 30.34 56.07SD 10.31 13.40 3.99

Possible solutionsM 40.99 38.20 56.26SD 7.78 12.64 3.34

RecommendationM 45.16 42.25 59.31SD 7.47 11.46 2.42

reported at the second session. A 2 (predicted vs.actual) £ 4 (project component) analysis of variance(ANOVA) was performed separately for the individualand group predictions. These analyses revealed thatactual completion times were signiWcantly longer thanboth the individual predictions, F (1, 14) D 70.83, p < .001,and the group predictions, F (1, 14) D 82.57, p < .001. Sub-sequent comparisons indicated that participants under-estimated their actual completion times for all fourcomponents of the project (all ps < .001 for both individ-ual and group predictions). In general, groups tended toWnish all four components of the project during the weekthat it was due, although they had expected to Wnishmuch earlier.

Next, to compare the individual and group predic-tions we conducted a 2 (prediction condition: individualvs. group) £ 4 (project component) within-subjectANOVA. A signiWcant main eVect of project componentindicated, unsurprisingly, that participants expected toWnish some sections of the project earlier than others,F (3, 12) D 44.73, p < .001. More importantly, a signiWcantmain eVect of prediction condition, F (1, 14) D 15.90,p < .001, indicated that participants generated more opti-mistic predictions through group discussion than theydid individually. There was a small and non-signiWcantprediction condition by project component interaction,F (3, 12) D 1.88, ns, suggesting that the eVects of groupdiscussion were similar across the four project compo-nents. Subsequent t tests performed separately for eachproject component indicated that the eVect of group dis-cussion was signiWcant for the individual sections involv-ing the statement of the problem (t [14] D 4.51, p < .001)and analysis of the problem (t [14] D 2.91, p < .05), butnot for the sections involving alternative solutions(t [14] D 1.23, p D .24) or a Wnal recommendation (t [14] D1.57, p D .14).

Additional t tests performed on the supplementarymeasures revealed no signiWcant diVerences betweenindividual and group predictions of how often the groupwould meet (Ms D 6.11 vs. 5.73, t [14] D .57, ns), or theextent to which the project would be enjoyable (Ms D4.92 vs. 5.20, t [14] D 1.25, ns), diYcult (Ms D 4.43 vs. 4.07,t [14] D 1.41, ns), and stressful (Ms D 4.27 vs. 4.33,t [14] D .29, ns). Comparisons of the predictions on thesemeasures with the post-task reports yielded only one sig-niWcant diVerence: Participants predicted they wouldhave more group meetings than they actually did(Ms D 6.11 vs. 4.67, t [14] D 3.94, p < .001).

In sum, the results were consistent with the initialhypothesis that people tend to underestimate completiontimes for group projects. The optimistic bias wasobtained for predictions generated by individual groupmembers, as well as predictions generated collabora-tively. One feature of the present study that may limit itsgeneralizability was that participants generated predic-tions for four task components. Conceivably, they


underestimated the time needed for each componentbecause their predictions were based on the notion thatthe components were independent, whereas during taskperformance they found that the components werelinked (e.g., they may have gone back to modify an ear-lier component when completing a later component).This concern is moderated, however, because the Wnalcompletion times were also underestimated. The resultsfor the Wnal completion times provide further evidence(Byram, 1997; Connolly & Dean, 1997), that the optimis-tic bias can occur even when predictors break down atask into sub-components (cf. Kruger & Evans, 2004).

The Wndings also provide preliminary evidence thatgroup discussion accentuates the optimistic bias, how-ever this conclusion remains tentative, due to limitationswith the simple pre-test/post-test design. We cannot ruleout the possibility that, even in the absence of group dis-cussion, the instruction to generate a second predictionand the additional time to consider the question mayresult in increased optimism.

Study 2

In the second study, we sought to replicate the patternof eVects obtained in Study 1 and rule out alternativeinterpretations. Thus, we moved to a more tightly con-trolled laboratory experiment. Groups were given a puz-zle task to complete in the laboratory and, to ensure thatthey desired early task completion, were provided withincentives for Wnishing quickly. Group members gener-ated task completion predictions under one of three dis-cussion conditions: consensus, consultation, and anindividual (no discussion) condition. In addition, we var-ied whether group members made initial private predic-tions prior to the discussion manipulation. An advantageof this 3 (discussion condition: consensus, consultation,individual) £ 2 (pre-test condition: pre-test vs. no pre-test) design is that it allowed us to assess whether makingan initial private prediction moderates the eVects ofgroup discussion. In addition, for half of the participants(those in the initial prediction condition) it was still possi-ble to make within-subject comparisons of predictionsgenerated before and after group discussion which pro-vide the most sensitive test of the eVects of group discus-sion. We predicted that group predictions would be moreoptimistic than either the initial predictions generatedindividually by the same group members (within-subjectcomparison) or the Wnal predictions generated in the indi-vidual condition (between-subject comparison).

Method

ParticipantsParticipants were 216 (141 females and 75 males)

undergraduate students who were recruited either

through the introductory psychology participant pool(and received course credit) or advertisements posted oncampus (and received payment of $10).

ProcedureParticipants were scheduled in groups of three for a

study examining how people work together on variousgroup tasks. On arrival at the laboratory, participantswere seated at individual tables identiWed by number andinformed that their group’s task would be to complete a100-piece jigsaw puzzle as quickly as possible. The exper-imenter also explained that participants in a pilot studyhad taken, on average, 14 min and 10 s to complete thepuzzle. As an incentive to work quickly, participantswere oVered an entry in a $150 lottery for every minutethey Wnished below this time.

The experimenter then gave each participant a puzzleto examine and asked them to complete an individualpre-task questionnaire. Participants Wrst indicated theirage and sex, and then reported how well they knew eachof the other participants (identiWed by table number) bychecking one of the following categories: completestranger, casual acquaintance, close friend, and veryclose relationship. Next participants answered severalquestions concerning their thoughts about the upcomingtask. On a scale ranging from 0 (not at all) to 10(extremely), they rated the extent to which: the taskwould be enjoyable, the task would be diYcult, theyhoped the task could be Wnished quickly, and they hadpersonal control over when the task would be Wnished.Participants also described in writing any aspects of thetask or situation that would hinder, and any aspects thatwould facilitate, a speedy completion time.3 As a mea-sure of their relevant experience, participants reportedhow often they had completed a jigsaw puzzle within thepast Wve years.

Next the pre-task questionnaire varied depending onwhether the group had been randomly assigned to eitherthe pre-test or no pre-test condition. Participants in thepre-test condition were asked to predict, as accurately aspossible, how many minutes/seconds their group wouldtake to complete the puzzle. They were also asked to listevery thought or idea that went through their minds asthey generated their prediction. Participants assigned tothe no pre-test condition did not complete these mea-sures.

3 For each study, preliminary analyses indicated that neither the gen-der composition of the groups nor the relationship status of groupmembers was signiWcantly related to any of the outcome variables, anddid not qualify the eVects of group discussion on predicted vs. actualcompletion times. Similarly, the supplementary measures of task ap-praisal, assessed prior to the group discussion manipulation, did notmoderate any of the eVects that we report. These factors are not dis-cussed further.


All participants were then asked to generate a Wnalprediction, as accurately as possible, of how long theirgroup would take to complete the puzzle, and instruc-tions were varied to create three discussion conditions.Participants in the consensus condition were asked toengage in group discussion for three minutes, and arriveat a single, consensus judgment of how long the puzzlewould take. Participants in the consultation conditionwere instructed to consult with their group for threeminutes, and then to report their Wnal prediction pri-vately on a recording sheet. To control for the passage oftime, participants in the individual condition were askedto spend three minutes thinking quietly about their pre-dictions, and then to report their Wnal prediction on arecording sheet. After recording their predictions, partic-ipants in all three conditions were asked to list thethoughts and ideas they had considered or discussed.Participants then completed the puzzle together and theexperimenter timed them with a stop watch. In sum-mary, the study consisted of a 2 (pre-test condition: pre-test vs. no pre-test) £ 3 (discussion condition: individual,consultation, and consensus) between subjects designand groups were randomly assigned to conditions.


A total of 72 three-person groups were distributedequally across the six experimental conditions. We againaveraged across the responses of individual group mem-bers and treated the group as the unit of analyses. Prelim-inary analyses indicated that, in general, participantsunderestimated their actual completion times. Initial pre-dictions in the pre-test condition were shorter (M D 13.28,SD D 1.67) than actual completion times (M D 15.00,SD D 4.11), t (35) D 2.57, p D .02. Final predictions werealso shorter (M D 12.53, SD D 1.86) than actual comple-tion times (M D 14.05, SD D 3.81), t (71) D 3.46, p < .001. Itis interesting to note that the Wnal predictions were alsoshorter than the mean completion time (based on pilottesting) provided to participants to help them make theirpredictions (M D 14.10), whereas the actual completiontime was virtually identical. On average, participantscould have made more accurate predictions by relying onthis normative information.

Intraclass correlations (icc’s) were computed on bothpreliminary and Wnal predictions to assess the degree ofsimilarity within groups. Note that the icc can be nega-tive (the lower bound is ¡1/n; Kenny et al., 1998) butgenerally ranges from 0 (groups are no more similarthan randomly selected individuals) to 1 (all individualswithin groups are identical on the variable of interest)and directly indicates the proportion of shared variancewithin groups (i.e., no squaring is necessary to yield a“variance explained” interpretation, Haggard, 1958). Asexpected, the preliminary predictions, which were madebefore any interaction took place in any group, were

associated with icc’s near 0 (¡.08 for the consensus con-dition, .04 for the consultation condition, and ¡.21 forthe individual condition). Consistent with the manipula-tions, after the interaction period within-group similaritywas very high for the group interaction conditions (icc’sof 1.0 for each of the consensus groups, as they wererequired to reach agreement; .55 and .62 for the no pre-test and pre-test consultation groups) but still small forthe groups that did not engage in discussion (.¡14 and¡.24 for the no pre-test and pre-test groups).

Changes in prediction within the pre-test conditionThe Wrst test of our primary hypothesis was per-

formed using the pre-test conditions only. See Table 2for means. We computed signed change scores indicatingthe diVerence between initial and Wnal predictions, andanalyzed these change scores using a one-way ANOVAwith one Helmert contrast pitting the group discussionconditions against the individual condition and a secondpitting the consensus condition against the consultationcondition.4 Consistent with our hypothesis, participantsin the discussion condition shifted toward more optimis-tic Wnal predictions (M change D ¡1.03, SD D 1.32),whereas those in the individual condition did not (Mchange D .25, SD D .74), t (32) D 3.33, p D .002. The sec-ond contrast indicated that the shift towards optimismwas greater in the consultation condition (M D ¡1.49,SD D 1.40) than in the consensus condition (M D ¡.57,SD D 1.11), t (32) D 3.20, p D .003.

EVects of group discussion and pre-test in full sampleWe examined our hypotheses in the full sample by

using an analysis of covariance design that controlledfor individual diVerences in task familiarity by using themean reported frequency of experience with puzzles foreach group as a control variable,5 controlled for experi-menter inXuences by entering experimenter (3 levels) as afactor, and examined the impact of group discussion andthe presence or absence of a pre-test in a core 3 £ 2 fac-torial design (see Table 3 for relevant means). Again, thegroup discussion factor was represented by two orthogo-nal contrasts: The Wrst provided a test of the primaryhypothesis by comparing the individual condition with

4 In Studies 2 and 3, to examine changes between measures taken atpre-test and post-test, we computed signed change scores and thencompared them across conditions. In each case, as an alternative ap-proach, we also performed a regression analysis on the post-test mea-sure using the pre-test measure as a control. The regression analysesyielded the same pattern of results and signiWcance levels as the analy-ses of change scores.

5 Task familiarity was included as a covariate because we anticipatedthat participants would vary considerably in their experience with puz-zles and that these individual diVerences would inXuence their expecta-tions and actual performance. Thus, inclusion of the covariate serves toaccount for error variance and provide a more powerful test of the hy-potheses.


the two group discussion conditions, and the secondcontrast compared the consultation and consensus con-ditions to determine whether these variants of group dis-cussion yielded diVerent eVects.

In the analysis of Wnal predictions, the covariate washighly signiWcant, F (1,54) D 9.79, p < .01, indicating thatgroups with more experienced members predicted theywould complete the puzzle more quickly; there was alsoa signiWcant eVect of experimenter, F (2, 54) D 3.23,p < .05, indicating that the degree of optimism varied byexperimenter. Importantly, the focused contrast analysisalso revealed, consistent with the hypothesis, that predic-tions were shorter in the group discussion conditions(M D 12.26, SD D 2.02) than in the individual condition(M D 13.08, SD D 1.37), t (64) D 2.07, p < .05. Predictionsdid not diVer signiWcantly across the consultation andconsensus conditions, t (64) < 1, ns. All interaction termswere small and nonsigniWcant, all ps > .20.

A parallel analysis of actual completion times revealeda marginally signiWcant eVect of experience, F (1,54)D3.18, pD .08, and a non-signiWcant eVect of experi-menter, p >.8. The absence of an experimenter eVect onactual completion times is noteworthy given that predic-tions had varied across experimenters. This pattern mayhave occurred because generating predictions involved agreater degree of subjective judgment—and thus wasmore readily inXuenced by idiosyncratic diVerences acrossexperimenters—than the actual performance of the task.The analysis also yielded a main eVect of the pre-test,

Table 2Initial and Wnal predictions (in minutes) within the pre-test conditionas a function of group discussion (Study 2)

Measure Discussion condition

Individual Consultation Consensus

Initial predictionM 12.89 13.87 13.07SD 1.48 1.94 1.51

Final predictionM 13.14 12.38 12.51SD 1.38 2.09 1.77

Change (Wnal ¡ initial)M 0.25 ¡1.49 ¡0.57SD 0.74 1.39 1.11

Table 3Final predictions and actual completion times (in minutes) by discus-sion condition (Study 2)



Final predictionM 13.08 12.39 12.13SD 1.37 2.13 1.94

Actual timeM 13.76 14.83 13.57SD 2.61 4.08 4.52

F (1,54) D4.04, p < .05, indicating longer completion timesin the pre-test conditions (MD15.00, SD D4.11) than inthe no pre-test conditions (MD13.10, SDD3.28). Thismain eVect was unexpected and is diYcult to interpretgiven that the pre-test did not aVect task predictions; theWnding was not replicated in the next experiment and thusis not discussed further. Important for our later hypothe-sis tests, there were no signiWcant diVerences in actualcompletion times between the group discussion conditionsand the individual condition (p >.90).

Although we have determined both through longitu-dinal and cross-sectional tests that group discussion ledto more optimistic predictions, a stronger test would beto control for the actual completion times. This analysisprovides a sensitive test of whether prediction bias—deWned as the residual variance in predicted times thatwas not accounted for by actual times—was aVected bygroup discussion. Thus, we performed an additionalANCOVA on Wnal predictions using completion timesand puzzle experience as covariates (experimenter wasdropped from the analysis as it was non-signiWcant onceagain). The analysis yielded the hypothesized eVect ofgroup discussion: After controlling for actual comple-tion times, the predicted completion times were shorterfor participants who engaged in group discussion thanthose who did not (adjusted Ms D 12.23 vs. 13.13),t (64) D 2.09, p < .05. The consultation and consensusconditions did not diVer signiWcantly (p > .75).

In summary, results from the full sample supportedthe hypothesis that group discussion would lead partici-pants to make more optimistic predictions, without hav-ing a corresponding eVect on actual completion times.The eVect of group discussion generalized across the twogroup discussion procedures, and was not moderated bywhether participants had previously made an individualprediction.

Coding of verbal protocolsThe open-ended thought listings were coded using

categories derived from previous research (Buehler et al.,1994), but modiWed to suit group tasks. The four maincategories included thoughts about future scenarios (e.g.,We can divide up the puzzle and each work on diVerentsections), relevant past experiences (e.g., I haven’t donepuzzles in a long time) group characteristics (e.g., We areintelligent girls so we can Wgure out the puzzle and com-plete it in a reasonable time), and task characteristics(e.g., I thought about the colors and how easily theywould be confused with one another). In each case, thethoughts could be either positive or negative (i.e., imply-ing a relatively short or long completion time, respec-tively) resulting in the eight content categories displayedin Table 4. Using a scale ranging from 0 (no focus) to 3(strong focus), a research assistant rated the extent towhich participants focused on each type of thought dur-ing the initial predictions (pre-test condition only) and


the Wnal predictions. We created a summary index ofpositive thought focus by summing across the four posi-tive items and subtracting the four negative ratings. Toestablish inter-rater reliability, a second rater coded theresponses from a subset of 36 groups; scores on the com-posite index were highly correlated across raters (r D .78;�D .88).

The mean ratings for thoughts underlying Wnal pre-dictions (see Table 4) indicated that participants focusedmost of their attention on positive future scenarios, thatis, their plans and strategies for completing the task suc-cessfully. These ratings were submitted to a one-wayANOVA with focused contrasts to determine whetherthey were aVected by group discussion. A signiWcanteVect of the Wrst contrast indicated that scores on thecomposite index of thought focus were signiWcantlyhigher in the group discussion conditions than in theindividual condition (Ms D 1.62 vs. .86), t (69) D 2.61,p D .01. Analyses of individual items indicated asexpected, that in comparison to participants in the indi-vidual condition, those in the group discussion condi-tions focused more on positive future scenarios(Ms D 1.41 vs. .78, t D 3.62, p D .001), more on positivegroup characteristics (Ms D .29 vs. .13, t D 1.76, p D .08),less on possible problems (Ms D .06 vs. .18, t D 2.52,p D .01), and less on negative task characteristics(Ms D .09 vs. 31, t D 3.01, p D .004). Unexpectedly, theyalso focused less on positive task characteristics(Ms D .24 vs. 63, t D 3.32, p D .001). On the whole, how-ever, these results suggest that group discussion led par-ticipants to focus selectively on information implying thetask could be completed quickly, and in particular ontheir plans and strategies for achieving this goal.

Analyses of change in thought focus performedwithin the pre-test condition oVered further support forthis interpretation. Scores on the overall index indicatedthat participants focused more exclusively on positiveinformation during the Wnal prediction than the initialprediction, and this shift was more pronounced in the

Table 4Mean ratings of thought categories by discussion condition (Study 2)

Category Discussion condition


Future scenariosPositive .78 1.36 1.45Negative .18 .07 .04

Past experiencesPositive .08 .06 .03Negative .14 .10 .06

Group qualitiesPositive .13 .39 .19Negative .11 .15 .11

Task characteristicsPositive .63 .26 .22Negative .31 .11 .08

combined discussion conditions than the individual con-dition (M change D 1.36 vs. 47), t (33) D 4.41, p < .001.Analyses of individual ratings revealed this pattern ofeVects, in which group discussion increased the focus onpositive (and decreased the focus on negative) informa-tion, for positive future scenarios (M change D 1.02 vs..22, t D 2.96, p D .006), negative future scenarios (Mchange D ¡.28 vs. ¡.03, t D 1.94, p D .06), positive groupcharacteristics (M change D .21 vs. ¡.08, t D 1.91, p D .06)and negative task characteristics (M change D .04 vs. .28,t D 2.44, p D .02).

Was the focus of participants’ thoughts as they gener-ated their Wnal predictions related to the optimism of thepredictions? Higher scores on the composite index ofthought focus were correlated with shorter predictedcompletion times (r D ¡.42, p < .001). More speciWcally,participants expected the puzzle would take less time themore they focused on positive future scenarios (r D ¡.25,p D .03) and positive characteristics of the group mem-bers (r D ¡.36, p D .002), and more time the more theyfocused on negative future scenarios (r D .21, p D .08) andnegative task characteristics (r D .28, p D .02). To deter-mine whether the eVect of group discussion on Wnal pre-dictions was mediated by the selective focus ofparticipants’ thoughts, we performed two hierarchicalregression analyses paralleling the ANOVAs presentedabove. The predicted completion times were Wrstregressed, as before, on the measure of relevant experi-ence, and group discussion, and then on the compositeindex of thought focus. When thought focus wasentered, the standardized regression coeYcient for thegroup discussion eVect was reduced from ¡.24,(t [66] D 2.26, p < .03) to ¡.08 (t [65] D 0.69, p > .49),whereas the eVect of thought focus was signiWcant(�D ¡.33, t [65] D 2.74, p < .01). This pattern suggests thatthe eVect of group discussion on predicted completiontimes was mediated by the focus of participants’thoughts as they generated their Wnal predictions(Sobel’s z D 2.36, p < .02 for a mediation test on theunstandardized coeYcients).

The results of the study supported each of our threehypotheses. First, participants underestimated how longthey would take to complete the puzzle task, eventhough they were provided with a useful source of distri-butional information—exactly how long previousgroups had typically taken. This Wnding appears to beconsistent with previous evidence (Buehler et al., 1997)that a goal to Wnish a task quickly (reinforced by a mon-etary incentive) can lead people to underweight relevantinformation about previous completion times. However,we cannot conclude that participants neglected the infor-mation about previous times in the present studybecause this information was not varied. Predictionsmay have been even more optimistic, as well as morevariable, in the absence of this normative data. Alterna-tively, the information may have actually fuelled


optimism if it heightened participants’ sense of inter-group competition or increased their motivation toenhance their own group (Polzer, Kramer, & Neale, 1997).

Second, predictions based on group discussion weremore optimistic (and more biased) than predictions gen-erated by individual group members. This Wnding repli-cates, using a more controlled experimental design, thepattern observed in Study 1. Importantly, the increasedoptimism occurred only in the group discussion condi-tions, indicating that it was not attributable to factorssuch as repeated questioning or additional time. Third,the results of thought listing measures suggest thatgroup discussion eVects were mediated by the focus ofparticipants’ thoughts as they arrived at their Wnal pre-dictions. Group discussion prompted participants tofocus more selectively on factors promoting successfultask completion (and to suppress thoughts about poten-tial impediments), and this selective focus contributed totheir optimistic predictions.

Of course, our ability to generalize from the presentWndings may be limited by the nature of the target task.The puzzle task was chosen because of features that itshared—in form if not in content—with many groupbased-tasks. For maximal success, every group memberneeded to contribute, they could select from a range ofpossible strategies, and they needed to co-ordinate theireVorts. However the task was also unlike many real-world tasks in that it was temporally compact and thusimmune to delays from unexpected disruptions, insuY-cient resources, competing demands, etc. In addition,unlike many real-world projects, group members couldwork simultaneously rather than waiting for the contri-butions of others. The external validity of the Wndingswould be reinforced if they were replicated with a morecomplex and longer-term target project completed out-side the laboratory.

Study 3

The purpose of the present study was to assess ourhypotheses using the same experimental design as inStudy 2, but with a more extensive, long-term target pro-ject. Participants were asked to complete a take-homeassignment with two other group members over a one-week period. The assignment required each groupmember to locate factual information, to forward thisinformation to other group members, and to submit aWnal project compiling all of the information. Becausecomponents of the project needed to be completedsequentially, group members were highly inter-depen-dent. The Wnal projects were submitted by e-mail, andthe computer records provided an objective measure ofcompletion time. As in the previous study, to ensure thatparticipants were motivated to complete the taskpromptly, we oVered incentives for early completion. We

expected that eVects obtained in Study 2 would general-ize to the longer-term, take-home project used in thepresent study.

Method

ParticipantsThe sample initially included 267 students, compris-

ing 89 three-person groups. Three groups were subse-quently excluded because they did not follow taskinstructions. The Wnal sample included 258 students (78males, 180 females) comprising 86 three-person groups.Participants were recruited from the introductory psy-chology participant pool and received course credit.

ProcedureParticipants were scheduled in groups of three for a

study examining how people think about and carry outcollaborative tasks. They were informed that they wouldbe asked to complete an assignment with two othergroup members outside the laboratory, and to make sev-eral judgments about the assignment in the initial groupsession. Upon arrival at the group session, participantswere seated at individual tables identiWed by number,and provided with detailed instructions for the take-home assignment. The assignment involved Wndinganswers to ten factual questions that would be drawnfrom a larger pool. The questions covered a wide rangeof topics (e.g., word deWnitions, history, geography, andcurrent events) and required highly speciWc answers thatwould not be common knowledge for most participants.Some answers could be found by searching the internetor reference books (e.g., What is Dizzie Gillespie’sproper name?) and others required that participants goto particular locations near campus (e.g., What color isthe sign on the Words Worth bookstore?). Participantswere shown a list of the kinds of questions that theirgroup would receive; however the actual assignment wasnot provided to participants until the day after their ini-tial session.

Further instructions for the assignment were designedto ensure that it required collaboration among all groupmembers. The experimenter explained that she woulde-mail the assignment the next morning to a designatedgroup member. Each participant was expected to answerat least two of the questions, and then forward theanswers by e-mail to the next group member. The lastgroup member was expected to submit the completedassignment—including the sequence of e-mail corre-spondence—to the experimenter. This process yielded anobjective measure of task completion time, and ensuredthat all group members contributed to the assignment.The assignment was considered complete if at least 90%of the solutions were correct; otherwise it was returnedto the group for revision. The assignment was due sevendays after the initial session. To provide participants


with an incentive to Wnish early, they were oVered a $1bonus for each day that the assignment was completedbefore its deadline.

After receiving the assignment instructions, partici-pants completed a pre-task questionnaire similar to thatof the previous study. Participants Wrst reported theirage and sex, and how well they knew each other. Nextparticipants rated, on a scale ranging from 0 (not at all)to 10 (extremely), the extent to which: the task would beenjoyable, the task would be diYcult, they hoped thetask could be Wnished quickly, and they had personalcontrol over when the task would be Wnished. Partici-pants also described in writing any aspects of the task orsituation that would hinder, and any aspects that wouldfacilitate, a speedy completion time.

Next, the questionnaire presented the pre-test andgroup discussion manipulations used in the previousstudy. Participants in the pre-test condition were askedto predict, as accurately as possible, the date/time theirgroup would Wnish the assignment, and to list theirthoughts as they generated the prediction. Participantsin to the no pre-test condition did not complete thesemeasures. All participants then generated a Wnal predic-tion, under one of the three discussion conditionsdescribed previously. After recording their predictions,participants were asked to list the thoughts and ideasthey had considered or discussed. Thus, the study againconsisted of a two 2 (pre-test condition: pre-test vs. nopre-test) £ 3 (discussion condition: individual, consulta-tion, and consensus) between subjects design and groupswere randomly assigned to condition.

The next morning the experimenter e-mailed theassignment to the designated group member. When eachassignment was submitted, the experimenter repliedwithin 24 h to indicate whether it was satisfactory. Ifthere was any problem with the assignment (e.g., morethan one incorrect answer), the experimenter returned itand explained the necessary changes. The experimenterrecorded the date and time that each assignment wasWnished.


The measures of predicted and actual completiontime (dates/times) were converted into the number ofdays (and fraction of days) from the initial session. Weagain averaged across individual group members andtreated the group as the unit of analysis. Preliminaryanalyses within the pre-test condition revealed no signiW-cant diVerence between the initial predictions (M D 2.03days, SD D .93) and actual completion times (M D 2.35days, SD D 2.38), t (43) < 1, ns. In their Wnal predictions,however, participants expected the assignment to beWnished about one day earlier (M D 1.48 days, SD D 1.04)than it actually was (M D 2.35 days, SD D 2.38),t (85) D 3.81, p < .001. Interestingly, participants pre-

dicted they would Wnish, and actually did Wnish, well inadvance of the seven day deadline, implying that thiswas a relatively “easy” task.

Once again, the preliminary predictions, which weremade before any interaction took place in any group,were associated with icc’s near 0 (.04 for the consensuscondition, .30 for the consultation condition, and ¡.05for the individual condition). After the interactionperiod within-group similarity was very high for thegroup interaction conditions (icc’s of 1.0 for each of theconsensus groups, as they were required to reach agree-ment; .91 and .79 for the no pre-test and pre-test consul-tation groups) but still small in the individual condition(.18 and ¡.14 for the no pre-test and pre-test groups).

Changes in prediction within the pre-test conditionAgain, we computed signed change scores indicating

the diVerence between initial and Wnal predictions (seeTable 5), and analyzed them using a one-way ANOVAwith one Helmert contrast pitting the two group discus-sion conditions against the individual condition and asecond pitting the consensus condition against the con-sultation condition. Consistent with our hypothesis, par-ticipants in the discussion condition shifted toward moreoptimistic Wnal predictions (M change D ¡.72, SD D .76),whereas those in the individual condition shifted little(M change D ¡.06, SD D .30), t (41) D 2.22, p D .003. Thesecond contrast indicated that the shift was no diVerentbetween the consultation condition (M D ¡.67, SD D .75)and the consensus condition (M D ¡.78, SD D .78),t (41) D 0.51, p > .6.

EVects of group discussion and pre-test in full sampleWe examined the impact of the group discussion and

pre-test variables in 3 £ 2 factorial ANOVAs (seeTable 6 for relevant means). Again, the group discussionfactor was represented by two orthogonal contrasts: TheWrst contrast compared the individual condition with thetwo group discussion conditions, and the second con-trast compared the consultation and consensus condi-tions.

Table 5Initial and Wnal predictions (in days) within the pre-test condition as afunction of group discussion (Study 3)



Initial predictionM 2.07 2.41 1.56SD .85 .99 .80

Final predictionM 2.01 1.63 .91SD .75 .93 .67

Change (Wnal ¡ initial)M ¡.06 ¡.78 ¡.67SD .30 .78 .75


The analysis of Wnal predictions revealed that partici-pants expected the assignment would take less time inthe discussion conditions (M D 1.28 days, SD D 1.10)than in the individual condition (M D 1.87 days,SD D .79), t (80) D 2.57, p D .01. The consultation andconsensus conditions did not diVer signiWcantly,t (80) D 1.59, p > .10. The interaction term also was notsigniWcant, t (80) < 1, ns, indicating that the eVect ofgroup discussion was not moderated by whether partici-pants had made an initial prediction.

The analysis of actual completion times yielded nosigniWcant eVects. Importantly, the assignment was notcompleted earlier by groups who engaged in discussion(M D 2.47 days, SD D 2.63) than those who did not(M D 2.48 days, SD D 2.11), t (80) < 1, ns. Likewise, com-pletion times did not diVer between the consultation andconsensus conditions, t (80) D 1.12, ns. The interactionalso was not signiWcant, t (80) < 1, ns. In general, partici-pants in all three conditions completed the assignmentabout two and a half days after receiving it, again,underlining the fact that this was a relatively easyassignment.

As in the previous study, a stronger test of ourhypothesis would be to control for the actual completiontimes. Thus, we again performed an ANCOVA on Wnalpredictions using completion times as a covariate. Aftercontrolling for actual completion times, the predictedcompletion times were shorter for participants whoengaged in group discussion than those who did not(adjusted Ms D 1.28 vs. 1.86), t (79) D 2.64, p < .02. Theconsultation and consensus conditions did not diVer sig-niWcantly, t (79) D 1.37, p > .15.

In sum, analyses performed on the full sample againrevealed that group discussion led to more optimisticpredictions, but did not have a corresponding eVect onactual completion times. The eVect of group discussionagain generalized across the two discussion procedures,and was not moderated by whether participants made anindividual prediction prior to discussion.

Coding of verbal protocolsThe thought listing responses were rated using the

same categories as in the previous study: future scenarios(e.g., If we split up the work amongst us, then there will

Table 6Final predictions and actual completion times (in days) by discussioncondition (Study 3)



Final predictionM 1.87 1.49 1.07SD .79 1.09 1.08

Actual timeM 2.48 2.84 2.12SD 2.11 2.51 2.73

not be that many questions per person), relevant pastexperiences (e.g., I’m usually very fast Wnding researchon the Internet), group characteristics (e.g., These peopleseem like hard workers and nice), and task features (e.g.,Some answers require walking 10–15 min, and some arediYcult). Within each main category, thoughts could beeither positive or negative (i.e., implying an early or latetask completion time, respectively). Using a scale rang-ing from 0 (no focus) to 3 (strong focus), a research assis-tant rated the degree to which participants focused oneach of the eight types of thoughts to generate their ini-tial and Wnal predictions. We again created a compositeindex of positive thought focus by summing across thepositive items and subtracting the negative items. A sec-ond rater coded the responses from a subset of 40groups; scores on the index were highly correlated acrossraters (r D .69; �D .81).

The ratings for Wnal predictions (see Table 7) indicatethat participants again focused most of their attentionon plans and strategies for completing the task success-fully. The ratings were also aVected by group discussion.Scores on the composite index were higher in the groupdiscussion conditions than in the individual condition(Ms D 1.90 vs. 1.10), t (83) D 3.69, p < 001. As in the previ-ous study, in comparison to participants in the individ-ual condition, those in the group discussion conditionsfocused more on positive future scenarios (Ms D 1.81 vs.1.14, t D 3.93, p < .001), and less on possible problems(Ms D .19 vs. .39, t D 2.13, p D .04). In the present study,however, no other ratings revealed signiWcant eVects ofgroup discussion.

One unpredicted pattern in the protocol data is note-worthy because it also appeared in the previous study.Participants in the consultation condition appeared tofocus more on positive group characteristics than thosein either the individual or consensus condition. A focuson positive group characteristics within the consultationcondition is consistent with our hypotheses, and we canonly speculate as to why a similar focus was not

Table 7Mean ratings of thought categories by discussion condition (Study 3)

Category Discussion condition


Future scenariosPositive 1.14 1.46 2.16Negative .39 .29 .11

Past experiencesPositive .03 .02 .02Negative .14 .10 .06

Group qualitiesPositive .10 .24 .03Negative .01 .05 .00

Task CharacteristicsPositive .31 .15 .18Negative .08 .00 .02


observed in the consensus condition. A key diVerencebetween these conditions is the requirement to reach asingle consensus judgment. Conceivably, this require-ment heightened the tendency of groups to focus onplanning out their tasks, and this focus detracted froman emphasis on group characteristics. Consistent withthis account, across both studies there was a somewhatstronger tendency (though not signiWcant) to focus onplans in the consensus group than in the consultationgroup. Within the pre-test condition, analyses of changeon the overall thought focus index again revealed thatparticipants in the group discussion conditions focusedmore exclusively on positive information during the Wnalprediction than during the initial prediction (Mchange D .85) whereas participants in the individual con-dition did not (M change D ¡.09), t (42) D 2.49, p D .02.Analyses of the individual thought categories revealedonly one signiWcant eVect. Participants in the group dis-cussion conditions increased their focus on positivefuture scenarios whereas those in the individual condi-tion did not (M change D .56 vs. ¡.24, t D 2.82, p D .007).

Correlational analyses again indicated that the focusof participants’ thoughts as they generated their Wnalpredictions was related to the optimism of the predic-tions. Groups with higher scores on the positive focusindex predicted they could Wnish in less time (r D ¡.42,p < .001). More speciWcally, groups expected the projectwould take less time if they focused more on future sce-narios and plans (r D ¡.27, p D .01) and more time if theyfocused on negative future scenarios (r D .36, p D .001)and negative group characteristics (r D .35, p D .001).

To test mediation, we again performed a hierarchicalregression analysis in which the predicted completiontimes were regressed Wrst on the group discussion vari-ables, and then on the composite index of thought focus.When the thought focus index was entered, the stan-dardized regression coeYcient for the group discussioneVect on prediction was reduced from ¡.27, (t [83] D 2.57,p D .01) to ¡.14 (t [82] D 1.32, ns) whereas the eVect ofthought focus was signiWcant (�D ¡.35, t [82] D 3.15,p D .002). This pattern suggests that the eVect of groupdiscussion on predicted completion times was mediatedby the focus of participants’ thoughts as they generatedtheir Wnal predictions (Sobel test on unstandardizedcoeYcients: z D 2.42, p D .02).

In sum, even though the task used in this study wasquite diVerent than those examined earlier, the resultssupported the three hypotheses. First, participants—butonly after group discussion—underestimated how longthey would take to complete the task. Unlike the previ-ous studies, individual predictions were not systemati-cally biased. Second, following from this contrast,predictions based on group discussion were more opti-mistic (and more biased) than predictions generated byindividual group members. Again, an increase in opti-mism over time occurred only in the group discussion

conditions, and not in the individual prediction condi-tion. Third, group discussion appeared to inXuence pre-dictions through its inXuence on the planning process,increasing the inclination to focus selectively on factorspromoting successful task completion and to suppressthoughts about potential impediments.

The series of tasks that made up the group projectwere less diYcult than we anticipated: two and a halfdays was generally suYcient to complete a task to whichwe had assigned a time limit of 7 days. However, the factthat the group discussion eVects were found even whenthe task did not give rise to an optimistic bias in the indi-vidual condition underlines the robustness and general-ity of the eVects.

As mentioned previously, we introduced the take-home assignment in Study 3 primarily to test whetherthe Study 2 Wndings would generalize to a more exten-sive, long-term project. Readers familiar with typologiesof group tasks (e.g., Steiner, 1972) will note that, in com-parison to the puzzle task used in Study 2, the take-homeassignment was more clearly divisible (i.e., it could bebroken down into sub-components), maximizing (i.e.,the quality of performance mattered), and conjunctive(i.e., all group members needed to contribute). Futureresearch should explore more systematically how diVer-ences in group tasks along these dimensions are relatedto forecasting accuracy. One possibility, is that conjunc-tive tasks (in which members must each make a separatecontribution) may be less prone to optimistic bias thanother tasks because people are inclined to think like“neutral observers” concerning others’ contributions.This may help to explain why the optimistic bias was lesspronounced in Study 3 than in previous studies.

General discussion

Clearly, neither group planning sessions nor groupactivities are panaceas for the generally observed opti-mistic bias in completion time forecasts. Across threestudies, we found a consistent optimistic bias in time pre-dictions for collaborative group projects, we found thatpredictions based on group discussion were more opti-mistic than individual predictions, and we found that theeVects of group discussion were mediated by individualgroup members’ thought focus at the time of prediction.Each of these three main Wndings is discussed in turnbelow.

Optimistic bias for group projects

The optimistic bias for group projects was robust. Inall three studies groups tended to predict that theywould Wnish collaborative tasks—ranging from shortlaboratory puzzles to projects carried out over severalweeks—earlier than they actually did. This Wnding


extends previous work on optimistic biases in planning(Buehler et al., 2002) to include both group tasks andgroup-based predictions. These extensions are valuablebecause people often make predictions interactively, andeVective temporal planning is critical to successful taskperformance in groups (Janicik & Bartel, 2003)

Although all three studies found an optimistic bias inthe group planning conditions, we cannot directly trans-late this Wnding into conclusions about the planning fal-lacy, per se. Recall that the planning fallacy is anoptimistic prediction for the future in the face of a gen-eral belief that the task usually takes longer thanexpected (Buehler et al., 1994; Kahneman & Tversky,1979). In Study 1, we have no direct measure of partici-pants’ beliefs about how often they usually completesimilar tasks (although in our previous studies of majorclass projects, students reported that past projects aretypically completed about 1 day before the deadline, andlater than initially anticipated). In Study 2, participantswere told the average time to complete the puzzle andgenerally predicted they would beat this time. In Study 3,participants had no prior experience with or informationabout the time such a task usually took. Thus, althoughthese Wndings have relevance for the planning fallacy ingroups, strictly speaking they deal with optimistic biasesin time prediction.

People’s predictions for collaborative tasks appearto resemble predictions concerning their own pro-jects—where optimistic bias is also widespread—ratherthan their predictions about others, which are rela-tively pessimistic (Buehler et al., 1994). Note, however,that it is diYcult to compare the present studies, whereboth individuals and groups predicted the outcome ofgroup projects, with past studies where actors andobservers predicted the outcome of individual projects.An important direction for future research will be tocompare more directly how and how well individualsmake predictions for group-based projects vs. individ-ual projects.

It is also worth considering how the present Wndingsrelate to previous research on group goal-setting, andparticularly studies examining self-set goals in group set-tings. Generally, group performance is improved whengroups collectively set challenging performance goals(Kerr & Tindale, 2004; Weingart, 1992; Weldon, Jehn, &Pradhan, 1991). Although self-set goals are conceptuallydistinct from the kinds of performance predictions weelicited, there is some evidence that a wide range of per-formance-related statements (including self-set goals,intentions, and performance predictions) can have simi-lar motivational eVects (Henry & Sniezek, 1993). Fromthe perspective of goal-setting research, then, it may besurprising that the diVerences in predictions across con-ditions did not translate into diVerences in actual taskperformance. Further research could beneWt fromexploring in group contexts the relations among various

types of goals (self-set, assigned, do-your-best) andgroup-based predictions to determine when and howthese performance-related statements have facilitatingeVects.

EVect of group discussion on optimistic bias

The second major Wnding was that predictions basedon group discussion were more optimistic than individ-ual predictions. Although the increase in optimism wasnot a large eVect, it was robust across three studies andtasks. The Wnding contributes to a growing body of stud-ies examining the contexts in which groups attenuate orexacerbate individual judgment biases (Kerr & Tindale,2004). Our Wndings are consistent with the “group accen-tuation eVect” and support the view that whether groupsattenuate or accentuate bias depends a great deal on theprevalence of the bias at the individual level (Hinsz et al.,1997; Tindale, 1993). According to this view, if somebias, error, or tendency predisposes individuals to pro-cess information in a particular way, then groups exag-gerate this tendency (for exceptions to this pattern, seeKerr et al., 1996).

A limitation of the present research is that it did notidentify, at a within groups level of analysis, the pro-cesses involved in moving from individual to consensusforecasts. Group decision researchers have developedsophisticated models (e.g., Social Judgment Scheme,Davis, 1996; Social Decision Scheme for Quantities,Hinsz, 1999; Revision and Weighting model, Sniezek &Henry, 1990) to describe the process of reaching groupconsensus. These models indicate the weights accordedto individual judgments in arriving at quantitative deci-sions. Our current designs oVer limited scope for suchanalyses. For example, Study 3, which featured the larg-est number of groups overall (86), had only 14 groups inthe consensus condition who had given preliminary indi-vidual responses. One relevant question in this case iswhether the group prediction could be Wt by the mostoptimistic group member (a form of extreme minorityinXuence). A paired t test comparing the most optimisticprediction with the resulting consensus judgmentshowed that this scheme could not be rejected(t (13) D 1.50, p > .15). However, the issue of power loomslarge here—the Wt of this scheme may well have beenrejected with a larger number of groups. A particularlyinteresting direction for future research on group fore-casting will be to examine decision schemes withingroups and identify how these relate to the accuracy orbias of group time estimates. According to a frameworkdeveloped by Kerr et al. (1996), and supported by com-puter simulations, whether groups exhibit more or lessbias than individuals depends upon not only the preva-lence of bias at the individual level, but also the strengthof the bias, the distribution of individual predictions,and the type of group decision process.


Mediation of group discussion eVects

Although we did not design our studies to model deci-sion schemes within groups, we did develop and testhypotheses concerning the impact of group discussionon participants’ informational focus during prediction.Consistent with our mediation hypotheses, group discus-sion prompted an increased focus on factors promotingsuccessful task completion, and this focus mediated theeVects of group discussion on prediction. This pattern isagain consistent with the view that group discussionincreased bias by exaggerating cognitive processes thatproduce bias at the individual level (e.g., Hinsz et al.,1997; Tindale, 1993). As in previous research, informa-tion weighted heavily by individuals seemed to beweighted yet more heavily by groups. More generally,the Wndings Wt in with the emerging conceptualization ofgroups as information processors (Hinsz et al., 1997)which implies that much can be learned about groupjudgment by examining how groups process relevantand available informational cues.

It is also possible that normative inXuences—involv-ing people’s desires to be viewed favorably by their col-laborators—may have contributed to group optimism(Gerard, 1963; Kahneman & Lovallo, 1993). Althoughwe did not measure normative processes, comparisonsbetween the consultation and consensus conditions pro-vides indirect evidence. If normative inXuence leads par-ticipants to make optimistic predictions, we might haveexpected that participants who were required to report aWnal prediction publicly (consensus condition) wouldmake more optimistic predictions than those allowed tomake a private Wnal prediction (consultation condition).However, in both Study 2 and Study 3, Wnal predictionsin the consultation and consensus conditions did notdiVer. This null eVect should be interpreted cautiously,because the consultation and consensus conditionsdiVered in more ways than just whether their Wnal pre-dictions were expressed privately or publicly, but it sug-gests that participants did not simply express optimismin groups that they did not endorse privately.

Generalizations, limitations, and future directions

In addition to the theoretical contribution of our Wnd-ings, discussed above, we believe they have considerableapplied value. Planners in organizational contexts oftendevelop their forecasts in team meetings, and are calledupon to justify their predictions on the basis of detailedstep-by-step plans. Even when individuals, such as pro-ject managers, render a Wnal prediction alone, they usu-ally interact with others to collect information, opinions,and advice (Heath & Gonzalez, 1995). Presumably peo-ple engage in consultation or discussion because theybelieve it will improve the accuracy of their forecasts.According to the results of our studies, however, group

discussion can actually exacerbate the tendency towardunrealistic predictions through an even greater tendencyto “plan for success”. These Wndings may be applicableto many collaborative work ventures that are similar inform to the projects examined here—that is, projects tobe completed collaboratively by groups who are moti-vated to Wnish early. Under such circumstances forecast-ers may be well advised to collect and aggregateindividual forecasts, instead of engaging in group discus-sion. It may also be beneWcial to consult neutral observ-ers, who appear to be less prone to optimistic biases(Buehler et al., 1994), rather than relying solely on pre-dictions from active contributors.

It would be premature to suggest that our Wndingswill generalize to most forecasting environments. A limi-tation of the studies is that they involved unfamiliargroups with no history of completing projects together.A more complete picture of group planning and predic-tion could be obtained by studying pre-existing workgroups with a shared history of relevant experience. Sev-eral other factors may also moderate the eVects, includ-ing: task characteristics, motivational factors, the social-relations among group members, and the structureimposed on communication. Further research is alsoneeded to examine the possible eVects of diVerences insocial relations among forecasters, examining qualitiessuch as interpersonal closeness (e.g., whether discussantsare strangers, friends, intimate partners) and groupcohesion (i.e., how much they feel like a group), as wellas diVerential status and expertise among discussants.

Finally, the accuracy of prediction may dependupon the techniques used to guide and structure com-munication among individuals (e.g., Plous, 1995; Sni-ezek, 1992). The present studies allowed free,unstructured discussion, and follow-up studies shouldexplore the eVects of various process interventions (e.g.,the Delphi Procedure, the Nominal Group Technique)that impose structure and constraints on the interac-tion process. Group intervention studies in otherdomains have demonstrated that quantitative esti-mates can be improved by process interventions(Henry, 1995; Henry, Kmet, Desrosiers, & Landa, 2002;Plous, 1995; Reagan-Cirincione, 1994) that may yieldsimilar beneWts for completion time forecasts.

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Collaborative planning and prediction: Does group discussion affect optimistic biases in time estimation?