1. EXPERIMENTAL DESIGNS Criteria for Experiments
Independent, Dependent, and Confounding Variables
Types of Experimental Designs
Threats to Internal Validity
Threats to External Validity

2. Criteria For Experimental Designs
Cause: experimenter manipulates a variable
Comparison: more than one condition
Control: extraneous variables are eliminated

3. Independent Variable variable manipulated by the experimenter levels
conditions

4. Dependent Variable
variable measured to assess the effect of the independent variable

5. Confounding Variable
variable other than IV and DV which changes between conditions
control variable: potential confounding variable that is controlled

6. Types of Experiments
between subjects
matched groups
within subjects

7. Threats to Internal Validity: Individual Differences
Systematic differences between individuals in different groups
Strategies
random assignment
matched groups
within subjects design

8. History Events outside the experiment
Most likely when conditions are measured at different times with long delays
Strategies
Decrease time between conditions
Add a control group measured at same times

9. Maturation Physical changes related to aging
Particular problem for within-subjects designs
Strategies
decrease time between measurements
add a control group measured at same times

10. Instrumentation Changes in the measuring instrument or equipment
Strategy
Use standardized administration

11. Attrition
Participants drop out of the study at different rates for the different conditions
Strategies
Check attrition rates across groups
Compare participants who drop out to those who stay in

12. Diffusion of Treatment
Information about the purpose of the study is shared with future participants
Strategies
Short time span between participants
Use debriefing to request that participants do not share information about the study

13. Demand Characteristics
Cues from the experimenter or research procedure about what behavior is desired
Strategy
Single-blind procedure

14. Experimenter Effects Experimenter’s expectations affect measurements
Strategy
Double-blind procedure

15. Floor and Ceiling Effects
Measuring instrument is not sensitive enough
Floor effects
Ceiling effects
Strategy
Check sensitivity of instrument prior to experiment

16. Regression to the Mean
When measured twice, scores on the second testing tend to be closer to the mean
Statistical phenomenon due to chance
Strategy
Don’t select participants for groups based on extreme scores
Use an equivalently selected control group that does not get the treatment

17. Order Effects Also called Testing or Repeated Testing
Effects of repeated measurements
Fatigue effects
Practice effects
Carryover effects
Strategy
Counterbalance order of conditions

18. How Counterbalancing Works
Change the order of conditions
Order effects will still exist but will affect all conditions equally
This prevents order effects from being confounding

19. Complete Counterbalancing
Each possible order of conditions is used for an equal number of subjects
If your conditions are A,B, and C, 1/6 of participants will get each order:
ABC CAB
ACB CBA
BAC BCA

20. Latin Square Counterbalancing
Each condition is presented in each position for an equal number of subjects
Controls for practice and fatigue effects

21. Example Latin Square 1st 2nd 3rd 4th 1/4 get A B C D 1/4 get B C D A
1/4 get C D A B
1/4 get D A B C

22. Balanced Latin Square
Latin square with additional requirement that each condition precedes and follows every other condition equally often
Controls practice and fatigue effects
Controls simple carryover effects (involving effect of a single condition)

23. Balanced Latin Square 1st 2nd 3rd 4th 1/4 get A B D C 1/4 get B C A D
1/4 get C D B A
1/4 get D A C B

24. Randomized Counterbalancing
Used when there are multiple stimuli tested for each condition
Put the stimuli in random order for each participant

25. Threats to External Validity
Unrepresentative Sample
use random or stratified random sampling
do exact or systematic replications
Artificiality
use a more realistic setting
do systematic or conceptual replications