CHAPTER 6Control Problems in

Experimental Research

Chapter 6. Control Problems in Experimental Research

Chapter Objectives

• Distinguish between-subjects designs from within-subjects designs

• Understand how random assignment can solve the equivalent groups problem in between-subjects designs

• Understand when matched random assignment should be used when attempting to create independent groups

Chapter Objectives

• Distinguish between progressive and carry-over effects in within-subjects designs

• Describe the various forms of counterbalancing

• Describe the specific types of between- and within-subjects designs that occur in developmental psychology, and understand the problems associated with each

• Describe how participant/experimenter bias can occur and how it can be controlled

Between-Subjects Designs• Comparison is between two different groups of subjects (each subject

receives one level of IV)

• Necessary when • Subjects in each condition have to be naïve• Barbara Helm study

• Subject variable (e.g., gender) is the IV

• Main problem to solve: creating equivalent groups

Level 1 Level 2

Creating Equivalent Groups• Random assignment• Each subject has equal chance of being assigned to any

group in the study • Spreads potential confounds equally through all groups• Accomplished through blocked random assignment

Creating Equivalent Groups• Random assignment• Each subject has equal chance of being assigned to any group

in the study • Spreads potential confounds equally through all groups• Accomplished through blocked random assignment

• Matching • Deliberate control over a potential confound• Use when• Small N per group might foil random assignment• Some matching variable correlates with DV• Measuring the matching variable is feasible

Within-Subjects Designs

• Also called repeated-measures designs (same subjects in every level of an IV)

• Comparison is within the same group of subjects• Used when comparisons within the same individual are essential

(e.g., perception studies)

• Removes possibility that differences between levels of the IV due to individual differences

Level 1 Level 2

Within-Subjects Designs

• Main problem to solve order effects• Progressive• Carry-over (harder to control)• Sequence A-B may yield differ carryover than the sequence

B-A

Controlling Order Effects• Counterbalancing• Altering the order of the experimental conditions

• Complete counterbalancing (all possible orders = x!)• Test participants in every possible different order at least

once• Works well with only a few conditions

• Partial counterbalancing• Random sample of all possible combinations is selected

Notice: Skip p219 “Testing more than once per condition” to end of p 223.

Methodological Control in Developmental Research

• Cross-sectional design• Between-subjects design• Potential for cohort effects• Worse with large age differences

• Longitudinal design• Within-subjects design• Potential for attrition difficulties

• Cohort sequential design• Combines cross-sectional and longitudinal

Problems with Biasing

• Experimenter bias• Experimenter expectations can influence subject

behavior

• Controlling for experimenter bias• Automating the procedure• Using a double blind procedure

Problems with Biasing • Subject bias• Hawthorne effect: Effect of knowing one is in a study• “Good” subjects• Participants tend to be cooperative, to please the researcher

• Evaluation apprehension• Participants tend to behave in ideal ways so as not to be evaluated

negatively• Demand characteristics• Cues giving away true purpose and study’s hypothesis

• Controlling for participant bias• Effective deception• Use of manipulation checks• Field research

Ethical Responsibilities of Participants

• Be responsible• Show up for scheduled appointments, or inform research

of cancellation

• Be cooperative• Behave professionally when participating in research

• Listen carefully• Ask questions if unsure of your rights or of what you are

asked to do

• Respect the researcher• Do not discuss study with others

• Be actively involved in debriefing• Help the researcher understand your experience

Lab PrepStroop Effect

• John Ridley Stroop (1935)

RED GREEN BLUE YELLOW

RED GREEN BLUE YELLOW

1

2

3

Lab PrepStroop Effect

• Modern-day Stroop Paradigm

RED GREEN BLUE YELLOW

congruent

incongruent

RED GREEN BLUE YELLOW

AutomaticityRelative Speed of Processing (“horse-race” model)

“Horse-Race” ModelWhen two processes occur in parallel, the faster one May interfere with the slower one, but not vice versa.

RED

“red”

“Horse-Race” ModelWhen two processes occur in parallel, the faster one May interfere with the slower one, but not vice versa.

RED

“red”

Is the “Horse-Race” Model Supported?

Congruent Incongruent

Name color

Read word

What would the horse race model predict?If prediction turns out to be true, we support the model.