An analysis of the subjective strain perceived by parcel-deliverers

ELSEVIER International Journal of Industrial Ergonomics 13 (1994) 15-23

I n d u s t r i a l E r g o n o m i c s

An analysis of the subjective strain perceived by parcel-deliverers

Martin Schiitte ~'*, Shuangsheng Ge a, Rainer Hertting-Thomasius b

a Institute for Occupational Health, University of Dortmund, Department of Ergonomics, Ardeystrasse 67, W-4600 Dortmund 1, Germany

t, Technical University of Berlin, Institute for Work-Science, Ernst-Reuter-Platz 10, W-IO00 Berlin 1~ Germany

(Received October 10, 1992; accepted in revised form March 17, 1993)

Abstract

The aim of the present investigation was to determine the level of strain perceived by drivers employed by a parcel-delivery company. A total of ten activities which are typical for the work sequence were first derived from a task analysis. The subjectively perceived levels of strain associated with the 10 activities were then determined using the method of paired comparisons. 18 experienced parcel-deliverers took part in the investigation. After an analysis of the judgement consistency and the level of concordance among the judges, both of which produced satisfactory results, the 10 activities were scaled unidimensionally. The calculated strain-related scale values are presented and work-design measures are discussed.

Relevance to industry

The present strain-related analyses of the tasks performed by parcel-deliverers provide indications of those work sections to which work-design measures should be applied. Consequently, the results can be used in redesigning parcel-delivery work.

Key words: Paired comparison; Perception of strain; Parcel service; Activities

1. Introduct ion

In a representative study carried out by the European Centre for Applied Economics Re- search, more than 5 5 % of the 10,000 companies from the industrial, commercial, and service sec- tors from 14 European countries indicated that they used courier, express or parcel services for the transportation of goods or documents (May and Kille, 1990). The reasons provided by the

* Corresponding author.

companies were the speed and reliability of such carriers as well as the special services they of- fered such as the so-called door-to-door overnight system guaranteeing delivery within 24 hours. The market volume of the courier, express and parcel services was approximately 1.8 thousand million units throughout Europe in 1989. This is predicted to almost double to approximately 3.2 thousand million by the year 2000 if moderate growth in demand is assumed (May and Kiile, 1990).

On account of the predicted expansion of this branch, an increase in the need for personnel

0169-8141/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0169-8141(93)E0061-R

16 M. Schiitte et aL /International Journal of Industrial Ergonomics 13 (1994) 15-23

and, consequently, in the number of workplaces, is to be expected. Since workplaces should always be planned and designed according to ergonomic criteria, load and strain-related investigations in this field are of considerable importance since they provide indications of design deficits. The load and strain foci occurring at existing workplaces are identified using so-called actual state analyses (Laurig, 1990; Luczak, 1982; Rohmert, 1983). The aim of the present study was therefore to determine the strain associated with the delivery tasks. It was carried out in a parcel-delivery company which has 63 warehouses nationally and employs approximately 2,100 drivers who provide the delivery service. The annual turnover of dis- patched items amounts to over 70 million units.

2. Load and strain

According to ISO 6385 (see also DIN 33405; ENV 26385), load is to be understood as describ- ing all external influences on man which result in changes in behaviour and subjective experience. The effects which result in a person in connection with his/her individual characteristics are de- fined as strain. The definition of the two terms corresponds to the distinction which is made in the load-strain concept between the conditions relating to the activity and the particular person- related effects (Rohmert, 1984). Increasingly, at- tention is focusing on those models which consider the ability of a person to adapt actively to the loads which occur (Cox, 1985; Gopher and Donchin, 1986; Hacker and Richter, 1980; Ki~nst- let, 1980; Reicherts, 1988; Udris, 1981). Accord- ingly, the resulting strain is not only determined by the load and individual characteristics, i.e. by a person's abilities and skills, but also by the individual adaptation processes employed to meet the demands made by the loads. The particular compensatory behaviour is, in turn, dependent on individual assessment processes relating to load and strain. It is therefore necessary to explicitly consider a person's subjective perceptions and assessments (Cox, 1985; Nachreiner, 1980) when analysing load and strain. In terms of methodology, procedures (for survey: O'Donnell and Egge- meier, 1986; Schi~tte, 1986) which record a per-

son's current state of wellbeing with reference to various psychological dimensions (e.g. effort, ex- ertion, work-performance capability) are suitable in principle since both situation-related and behaviour-related assessments are included in the individual perception of wellbeing (Kiinstler, 1980; Nitsch, 1976). However, such methods only permit statements about the structure of an individual's experience of strain.

By contrast, work design requires specific, activity-related statements about the load and strain involved in the performance of the given task in order to be able to derive indications about those activities to which work-design measures should be applied as a matter of priority. Consequently, the present investigation aimed to analyse the level of strain associated with individual activities in a work sequence (see also Lindsay and Evans, 1974).

3. The method of comparative judgement

Subjective scaling methods are applied here since they have proven particularly appropriate in the unidimensional, quantitative recording of complex characteristics (Lienert, 1978). A pre- condition for their use is the ability of judges to integrate the various components with relevance to their experience of strain into a strain-related total judgement (Weber, 1985). The method of comparative judgement, previously employed in the context of work-psychological investigations (e.g. Bartenwerfer, 1969; Burkardt and Weber, 1985; K6tter, 1963; Luczak et al., 1986; SchiRte, 1989, 1990), was the preferred method here. It requires that a collective of judges possessing the highest possible degree of competence, i.e. famil- iarity with the particular task, selects from the activities arranged in pairs the one which results in the highest level of strain. According to the law of comparative judgement, such a higher/lower decision can be interpreted as a comparison effected "internally" in a person, describable for- mally as the difference between the reactions to the work-sequence activities in each case (Thur- stone, 1927). Scale values which reflect the subjectively perceived level of strain associated with the individual activities can then be calculated

M. Schiitte et al. /International Journal of Industrial Ergonomics 13 (1994) 15-23 17

from the percentage-based frequencies with which particular activities are preferred to others (Guil- ford, 1954; Sixtl, 1982; Torgerson, 1958).

4. Task description

A task analysis must be undertaken prior to a strain-related analysis of work activities. The analysis was based on the task descriptions for the parcel-deliverers formulated by the company in which the study was conducted. The tasks centred essentially on three areas, namely the preparation for the journey, the delivery of the item, and the post-delivery phase.

4.1. Preparation for the journey

The parcel-deliverer first has to collect the vehicle documents from the administrative offices at the warehouse and insert the disc into the vehicle's tachograph. After ascertaining details about the journey the deliverer must sort the parcels on an address basis and record them in a so-called orientation list. Valuable and express items, which are subject to special security, are then handed out to him. Subsequently, the cards for the returns, which include the addresses of the customers from whom items must be col- lected, have to be picked up from the administrative offices. The vehicle is then loaded.

4.2. Delivery of the items

The time of delivery depends on traffic conditions (e.g. traffic jams). When the driver reaches the customer's address he must first find a parking space. The relevant delivery item is taken from the storage area of the vehicle, which is subsequently locked. The item is then carried to the front door. Once the customer has answered the door, the driver hands over the item, records the data relating to the item on a receipt sheet and gets the customer to sign in confirmation of receipt of the item. If the customer has got returns, the driver must hand over a completed receipt for the returns and record the customer's name and the number of returns in a list of the day's returns.

4.3. Post-delivery tasks

After returning to the warehouse, a journey which is also subject to potential delays as a result of commuter traffic, the parcel-deliverer has to book the returns and unload the vehicle. The returns statement must then be checked by a member of the warehouse staff, the disc must be removed from the tachograph, and the vehicle parked.

5. Selection of the activities

The activities to be assessed by the parcel-deliverers were selected in consideration of the total range of tasks, from the preparation for the journey and the delivery to the tasks subsequent to the journey. One additional criterion was that the assessment had to be realizable in terms of time. A total of 10 work-sequence activities appeared sensible (see Table 1). However, the types of load associated with the 10 activities varied.

For example, mainly physical loads occurred in the activities in which manual load handling pre- dominates, i.e. "spreading out the items and entering them in the orientation list", "loading the vehicle", "carrying the item from the vehicle to the customer", "unloading the vehicle in the warehouse". By contrast, activities which basically involved the absorption, processing and conver- sion of information, i.e. all of those activities

Table 1 Description of the 10 selected sequence activities

Subtask Description of the sequence activities

Preparation for Spreading out the items and entering the journey them in the orientation list

Drawing up the journey schedule Loading the vehicle

Delivery of Stuck in a traffic jam the item Looking for a parking space

Carrying the item from the vehicle to the customer

Getting confirmation of receipt of the item from the customer

Filling in a return

Post-delivery Doing the accounts relevant to the journey tasks Unloading the vehicle in the warehouse

1 8 M. Schiitte et al. / International Journal of lndustrial Ergonomics 13 (1994) 15-23

associated with the documentation of the delivery of the item such as "drawing up the journey schedule", "getting confirmation of receipt of the item from the customer", "filling in a return", "doing the accounts relevant to the journey", represented psychological loads. Furthermore, there were also activities such as "stuck in a traffic jam" and "looking for a parking space" which involved an amount of time that cannot be estimated by the parcel-deliverer and, consequently, had an effect on the times of further deliveries.

1 O0

%

~= = 75 : . -

Q. cr 50

0 0,80 0,85 0,90 0,95 1,00

Kendall-Zeta

Fig. 1. Cumula ted d is t r ibut ion o f the coeff ic ients o f consis-

tency ( N = 18).

6. Sample

The investigations were conducted in three different warehouses belonging to the parcel-delivery company. A total of 18 parcel-deliverers were questioned. Their ages ranged between 22 and 49 years (M = 31.9 years, SD = 6.2 years) and their job experience amounted to between 6 and 185 months ( M = 47.8 months, SD = 46.7 months). Since, according to information provided by the company, training of the parcel-deliverers takes about 3 months, the persons questioned could be regarded as experienced workers.

7. Conducting the investigation

The 10 activities were combined to form a total of 45 pairs. The parcel-deliverers were in- structed to identify in each case the item associated with the highest level of strain. The sequence in which the pairs were presented to them was derived by applying a methodological approach which allows the determination of a so- called optimal presentation sequence (Ross, 1934). This approach served primarily to avoid regularities in the presentation of the activities, and to exclude the possibility of pairs sharing a common activity following each other directly.

8. Results

8.1. Analysis of the judgement consistency

A quantitative determination of the level of strain associated with the individual activities was

only possible on the basis of paired comparisons if it could be assumed that the parcel-deliverers were in fact able to arrange the 10 activities according to a h igher / lower relationship. This was examined by analysing the individual response behaviour for self-contradictory judgement linkages known as circular triads. The level of consistency achieved in each case could be described via a characteristic variable known as Kendall's Zeta. This coefficient could vary between "0" and "1" with "0" representing com- plete inconsistency in the assessment of the sequence activities and "1" indicating that the assessments were completely free from contradic- tions (Kendall and Babington Smith, 1940; Kendall, 1975). The judgement consistencies determined for the 18 parcel-deliverers were significant (chi-square > 37.57, p < 0.01, df = 20) and varied in total between 0.80 and 1.0, with over 50% of the subjects achieving a Kendall Zeta greater than 0.9 (see Fig. 1). Consequently, the assumption of mainly transitive response behaviour among those questioned was justifiable.

8.2. Analysis of the concordance among judges

The response behaviour among the parcel-deliverers must be as similar as possible if a quantitative determination of the level of strain associated with the individual sequence activities should be undertaken on the basis of the assessments of the total sample. To verify this, the level of concordance among the judges, ascertainable via the so-called Kendall W, was determined. This coeffi-

M. Schiitte et al. / International Journal of lndustrial Ergonomics 13 (1994) 15-23 19

cient could take values between "0" and "1", with "0" representing no concordance and "1" the maximum concordance (Kendall, 1975). The calculated Kendall W value amounted to 0.55 for the assessments of the 18 parcel-deliverers (chi- square = 88.54, dr= 9, p < 0.01). It could therefore be assumed that the concordance among the judges was significant. On account of the satisfactory level of concordance among the judges, unidimensional scaling of the sequence activities was undertaken for the total group comprising 18 parcel-deliverers.

8.3. Scaling

The sequence activities were scaled according to a unidimensional linear model corresponding to Torgerson's condition C (Torgerson, 1958). The quality of the scale values derived in this way could be determined graphically by comparing the preferences in per cent as predicted on the basis of the scaling model with those which actually occur (see Fig. 2). The initial data could be reproduced using the scaling solution determined here. An analysis to test the statistical signifi- cance of the data fit was conducted in order to confirm these findings (Gulliksen and Tukey, 1958). Three test variables had to be calculated, namely the so-called chi-square value "total", which was derived from the deviations of the observed percentage preferences from the total mean value, the chi-square value "linear", which was formed from the differences between the reproduced preferences and the total mean value

" 100

oio

~ 6o .E

~, 60

~ 4o

n 0 0:

0 20

- - ,~11" N

- - . . , . 7 •

40 60 50 % 100

Reproduced preferencies in per cent

Fig. 2. Compar i son of the obse rved and p red ic t ed p re fe r ences in pe r cent ( N = 18).

0.50

Loading the vehicle 0.47 Carrying the item from the vehicle 0.46 to the customer

0.44 Stuck in a traffic jam

0.40 Spreading out the items and entering them in the orientation l ist

0.36 Looking for a parking space

0,22 Unloading the vehicle in the warehouse

0.16 Doing the accounts relevant to the journey

0.09 Gett ing confirmation of receipt of the item from the customer

0.04 - Drawing up the journey schedule

0.O0 Filling in a return

Fig. 3. S t ra in - re la ted scale va lues for the 10 sequence activi-

t ies ( N = 18).

as well as the chi-square value "discrepancy", calculated from the differences between the observed and reproduced preferences. The significant chi-square value "total" (chi-square = 498.72, dr= 45, p < 0.01) demonstrated that the individual sequence activities actually were assessed differently according to the level of strain associated with them. The data also contained a marked "l inear" component (chi-square l inear-- 460.24, dr= 9, p < 0.01). It was not possible to detect significant discrepancies between the reproduced values and those which were determined empirically (chi-square discrepancy = 45.22, df = 36, p > 0.01). In order to establish the quality of the scaling solution, the percentage of the variance in the observed data which was explainable on the basis of the model was also determined. It amounted here to 89%. The reliability of the scale values was therefore calculated as 0.91. Consequently, the selected scaling model appeared appropriate for the data.

20 M. Schiitte et al. / International Journal of lndustrial Ergonomics 13 (1994) 15-23

8.4. Scaring results

The calculated normed scale values (Sixtl, 1982) for the strain levels of the 10 sequence activities are provided in Fig. 3.

The activity with the lowest number of preferences ("filling in a return") was arbitrarily chosen as the zero point. It was revealed that the sequence activities associated to a greater extent with physical load, "loading the vehicle", "carrying the item from the vehicle to the customer" as well as "spreading out the items and entering them in the orientation list" were, with scale values between 0.40 and 0.47, assessed as representing a high level of strain. Similarily the activities representing primarily psychological loads "stuck in a traffic jam" and "looking for a parking space" had high strain. By contrast, the other activities which were more psychological in nature, those connected with the documentation of the delivery of the items ("filling in a return", "drawing up the journey schedule", "getting confirmation of receipt of the item from the customer", "doing the accounts relevant to the journey"), were assessed, with scale values up to 0.16, as having low strain levels. The activity with greater physical demands, "unloading the vehicle in the warehouse", which was assigned a scale value of 0.22, represented low strain.

9. Discussion

Previously available methods for determining the experience of strain, such as the mood-word inventory (Bottenberg, 1970), the multidimensional mood questionnaire (Hecheltjen and Mertesdorf, 1973), the scale of self-being (Nitsch, 1976), the adjective scales for the assessment of mood (Hampel, 1977), the adjective-based word list (Janke and Debus, 1978), the questionnaire on the experience of load (Ktinstler, 1980) or the modified load course test (Schiitte, 1988) consist, as a rule, of a list of mood-related adjectives which the judges have to assess on the basis of the extent to which the individual items apply to their present state. The methods are mostly multidimensional. They allow the experience of strain to be recorded on scales, usually derived factor-

analytically, which represent the quality of experience such as effort, work-performance capability, etc. Statements about the fundamental necessity for work-design measures can be derived from the adverse effects on the state of wellbeing which are identified using such methods. However, the methods do not provide any information about the aspects of the work task which may have caused the changes in the state of wellbeing. This is particularly true in cases where, as in so-called actual state analyses, only one design variant was examined. The approach selected here did not therefore aim to determine states of wellbeing in a differentiated manner. Instead, the intention was to provide a detailed determination of the level of strain associated with the individual work-sequence activities in order to identify more easily those activities to which work-design measures should be primarily applied. Since, however, the analysis is always dependent on the work-sequence activities, it proves difficult to make comparisons between the strain occurring at different workplaces. It is also not possible to derive statements about the psychological structure of the experience of strain in each case.

The paired comparison method used in the present investigation assumes that the judges assess the activities presented to them according to a constant measure, i.e. transitively, throughout. This postulate can be regarded as fulfilled if the answers are consistent, that is to say, free from contradiction. The coefficients of consistency determined for the 18 parcel-deliverers were significant in each case and numerically of an order which justifies the assumption of an individually stable assessment measure. Furthermore, the method used here to analyse the judgement consistency can be compared with the concept of " inner consistency" which was developed for use in test psychology for the determination of reliability (Lienert, 1978). Kendall's Zeta can therefore also be understood as a measure of reliability. The judgement reliability of the sample under investigation exceeded the lower limit value of 0.7 which is recommended for standardized psychological tests (Lienert, 1969). Since intransitivities occur especially when the work-sequence activities do not, or hardly, differ from each other in

M. Schiitte et al. / International Journal of Industrial Ergonomics 13 (1994) 15-23 21

terms of the perceived level of strain (Hill, 1953), the analysis of consistency additionally permits conclusions about whether the strain-related perception of the work task actually reveals the differentiation assumed in the subdivision of the work sequence into activities. The level of judgement consistency achieved in each individual case therefore also indirectly provides an indication of the extent to which the subdivision of the work sequence corresponds to the psychological perception of the work sequence. With values between 0.8 and 1.0, the coefficients of consistency ascertained here varied within a range which justified the conclusion that the predetermined subdivision of the work sequence largely corre- sponded to the subjectively perceived differentiation.

Even though the scalings were only based on a global assessment of the 10 activities undertaken with regard to the aspect "strain", the satisfactory level of concordance among the judges indicated that the assessment characteristic "strain" had been interpreted interindividually in a relatively homogeneous manner. Consequently, the derived scale values can be regarded as representative of the group comprising the 18 parcel-deliverers. The results of the analysis of concordance can be additionally used to decide whether group-related (in the case of high concordance) or worker-specific (low concordance) design measures should be undertaken. Since there were only small differences overall between the parcel-deliverers in their assessment of the strain associated with the activities, the application of group-related measures appears fundamentally permissible.

However, the derived scale values only provide information about the differences in terms of the level of strain between the individual work-sequence activities. It is not possible to make statements regarding their absolute level. Further- more, the effect of variables which also influence strain, such as the duration of the individual activities, the order in which they occur, etc., cannot be estimated precisely (Hacker and Richter, 1980; Laurig, 1990; Sch6npflug, 1987).

In spite of the limitations already cited, the approach presented here can be regarded as a

sensible addition to existing inquiry methods which primarily record the different qualities of the subjective experience of strain since it provides more precise indications of the aspects of the work sequence to which changes must be made. In addition, the method can also be employed within the framework of preliminary stud- ies as a screening method for the determination of load and strain foci.

I0. Practical conclusions

Taking the present findings as a starting point, there are five activities to which work-design measures should be applied as a matter of priority, namely loading the vehicle, carrying the item, delays due to traffic jams, spreading out the items, and searching for a parking space. Prioritization was undertaken on the basis of the level of strain determined for each of the individual activities.

The cause of the high level of strain during the loading of the vehicle should be examined first. In the company involved in the current investigation, loading was effected manually. As a rule, a warehouse worker handed the delivery item over to the parcel-deliverer who was in the loading area of the vehicle. The strain which occurred was, therefore, dependent on the number and mass of the delivery items, the posture adopted in the loading process, and the duration of the activity.

The energetic load on the parcel-deliverers could be reduced by providing suitable loading and manipulation aids. This load could be avoided completely, however, if the vehicle was handed over already loaded. This, though, presupposes that the journey schedule is no longer left to the parcel-deliverers but, instead, is effected in ad- vance (for example by means of an EDP-based journey-scheduling system). It must be ensured here that the parcel-deliverers can find the individual delivery items in the vehicle without diffi- culty. This would involve providing them with information about the items to be delivered at the individual stops and their location in the vehicle in addition to the journey schedule. Ac- cordingly, the journey schedule would have to contain both the customer's address and a code

22 M Schiitte et aL / International Journal of lndustrial Ergonomics 13 (1994) 15-23

by means of which the particular delivery items could be identified. The search for the delivery items in the vehicle could be facilitated by equip- ping the vehicles with a system of precoded shelves on which the items with the corresponding code are stored. A fur ther advantage here is that the parcel-deliverers would be able to find the items relatively easily even if there were deviations from the journey schedule provided to them.

The strain connected with carrying the items from the vehicle to the customer depended on the number and mass of the delivery items which had to be moved as well as on the distance to be covered and the gradient. It can be assumed that personal characteristics, particularly the physical per formance capability, but also the lifting and carrying techniques employed, had a modera t ing influence here. Design measures can only be pro- posed to a limited extent because the distances to be covered by the parcel-deliverers depend on the part icular local conditions. However, the parcel-deliverers also had to handle heavy loads. Consequently, it would appear sensible to provide training measures in order to familiarize them with lifting techniques which are acceptable in terms of work physiology or biomechanics (Ar- bei tsgemeinschaft der Eisen- und Metall-Berufs- genossenschaften, 1991 - t eam belonging to the t rade association with responsibility for industrial injuries insurance in the iron and metal-working industries) since an indirect reduct ion in the physical loads which occur can be expected as a result.

The loads attr ibutable to delays (e.g. traffic jams) are above all due to the time pressure occurr ing when, for example, delivery of items in pedestr ian zones is only permit ted during particular periods or when the cus tomer is only con- tactable at part icular times. In such cases it is hardly possible to relieve the time pressure connected with the delivery by applying work-design measures.

The strain during the prepara t ion phase in the section "spreading out the items and entering them in the or ientat ion list" is essentially due to the unfavourable posture which was adopted. As a rule, the parcel-deliverer spread the items out on the ground and, consequently, had to work in a predominant ly s tooped posture. A reduct ion in

load could be achieved through the provision of appropr ia te sorting and manipulat ion aids such as tables and conveyors.

The strain associated with searching for a parking space - in part icular in city centres - also results f rom time pressure. This could be reduced if parcel-deliverers were allowed to park in a so-called second row. Accordingly, the only recommendat ion which can be made here is for the company to make greater efforts to acquire such authorizat ion f rom the relevant authorities.

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Documents

An analysis of the subjective strain perceived by parcel-deliverers