Experimental Design: Between and within factors

1. Experimental Design: Between and within factors Psych 231: Research Methods in Psychology

2. Announcements • Exam 2 coming up (Nov. 1) • In labs • Turn in methods sections for group projects • Turn in IRB form (in PIP packet) • Be prepared to pilot your studies next week

3. Between versus Within Subjects Designs • Between subjects designs • Each participant participates in one-and-only-one condition of the experiment. • Within subjects designs • all participants participate in all of the conditions of the experiment.

4. Example • What is the effect of presenting words in color on memory for those words? • So you present lists of words for recall either in color or in black-and-white.

5. levels • So each of the participants is in only one level of the IV 1 Between factor - 2 levels Clock Chair Cab Colored words participants Test BW words Clock Chair Cab

6. levels participants Colored words BW words Test Test • So all of the participants are in both levels of the IV 1 Within factor - 2 levels Club Chain Cat Clock Chair Cab

7. Between subjects designs. • Advantages: • Independence of groups (levels of the IV) • Harder to guess what the experiment is about without experiencing the other levels of IV • exposure to different levels of the independent variable(s) cannot “contaminate” the dependent variable • No order effects to worry about • Counterbalancing is not required • Sometimes this is a ‘must,’ because you can’t reverse the effects of prior exposure to other levels of the IV

8. Colored words Test participants BW words Between subjects designs • Disadvantages • Individual differences between the people in the groups • Non-Equivalentgroups • Excessive variability

9. Individual differences • Non- Equivalent groups • The groups may differ not only because of the IV, but also because the groups are composed of different individuals

10. Dealing with Individual Differences • Equivalent groups • Created equally - use the same process to create both groups • Treated equally - keep the experience as similar as possible for the two groups • Composed of equivalent individuals • Random assignment to groups • Matching groups - match each individuals in one group to an individual in the other group on relevant characteristics

11. matched matched matched matched Matching groups Group A Group B

12. NR NR R R NR other other exp Individual differences • Excessive variability due to individual differences • Harder to detect the effect of the IV if there is one

13. Within subjects designs • Advantages: • Don’t have to worry about individual differences • Same people in all the conditions • Variability between groups is smaller (statistical advantage) • Fewer participants are required

14. Within subjects designs • Disadvantages • Order effects: • Carry-over effects • Progressive error • Counterbalancing is probably necessary • Range effects

15. Order effects • Carry-over effects • Transfer between conditions is possible • Effects may persist from one condition into another • e.g. Alcohol vs no alcohol experiment on the effects on hand-eye coordination. Hard to know how long the effects of alcohol may persist.

16. Order effects • Progressive error • Practice effects – improvement due to repeated practice • Fatigue effects – performance deteriorates as participants get bored, tired, distracted

17. Dealing with order effects • Counterbalancing is probably necessary • This is used to control for “order effects” • Ideally, use every possible order • (n!, e.g., AB = 2! = 2 orders; ABC = 3! = 6 orders, ABCD = 4! = 24 orders, etc). • All counterbalancing assumes Symmetrical Transfer • the assumption that AB and BA have reverse effects and thus cancel out in a counterbalanced design

18. Colored words BW words Test Test participants BW words Colored words Test Test Counterbalancing • Simple case • Two conditions A & B • Two counterbalanced orders: • AB • BA

19. Counterbalancing • Often it is not practical to use every possible ordering • Partial counterbalancing • Latin square designs – a form of partial counterbalancing, so that each group of trials occur in each position an equal number of times

20. A B C D Order 1 B C D A Order 2 C D A B Order 3 D A B C Order 4 Partial counterbalancing • Example: consider four conditions • Recall: ABCD = 4! = 24 possible orders 1) Unbalanced Latin square: each condition appears in each position

21. Partial counterbalancing • 2) Balanced Latin square: each condition appears before and after all others

22. Within subjects designs • Range effects – (context effects) can cause a problem • The range of values for your levels may impact performance (typically best performance in middle of range). • Since all the participants get the full range of possible values, they may “adapt” their performance (the DV) to this range.

23. Mixed factorial designs • Mixed designs • This only works with factorial (multi-factor) designs • Treat some factors as within-subjects (participants get all levels of that factor) and others as between-subjects (each level of this factor gets a different group of participants).