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 (Oct. 31) • In labs • Turn in methods & Appendix sections for group projects • Turn in IRB form (in PIP packet) • Be prepared to pilot your studies next week

3. 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. Clock Chair Cab Clock Chair Cab • Two different designs to examine this question

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

5. levels participants Colored words BW words Test Test 1 Within factor - 2 levels Clock Chair Cab Clock Chair Cab • So all of the participants are in both levels of the IV • Sometimes called “repeated measures” design

6. participants Colored words Colored words BW words Test Test participants Test BW words Between vs. 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.

7. participants Colored words Colored words BW words Test Test participants Test BW words Between vs. 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.

8. Colored words participants Test BW words 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

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

10. Colored words participants Test BW words R R NR exp Difference detector Individual differences • Excessive variability due to individual differences • Harder to detect the effect of the IV if there is one

11. Colored words Test participants BW words Colored words participants Test BW words 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

12. Colored words participants Test BW words Dealing with Individual Differences • Strive for 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 - eliminate bias • Matching groups - match each individuals in one group to an individual in the other group on relevant characteristics

13. matched matched matched matched Red Short 21yrs Colored words participants Blue tall 23yrs Test BW words Green average 22yrs Brown tall 22yrs Matching groups Group A Group B • Matched groups • Trying to create equivalent groups • Also trying to reduce some of the overall variability • Eliminating variability from the variables that you matched people on Red Short 21yrs Blue tall 23yrs Green average 22yrs Color Height Age Brown tall 22yrs

14. participants Colored words Colored words BW words Test Test participants Test BW words Between vs. 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.

15. participants Colored words BW words Test Test 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

16. participants Colored words BW words Test Test Within subjects designs • Disadvantages • Order effects: • Carry-over effects • Progressive error • Counterbalancing is probably necessary • Range effects

17. participants Colored words BW words Test Test Condition 1 Condition 2 test test 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. How long do we wait for the effects to wear off?

18. participants Colored words BW words Test Test Order effects • Progressive error • Practice effects – improvement due to repeated practice • Fatigue effects – performance deteriorates as participants get bored, tired, distracted

19. participants Colored words BW words Test Test 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

20. participants Colored words BW words Test Test 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

21. participants Colored words BW words Test Test 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

22. participants Colored words BW words Test Test 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

23. participants Colored words BW words Test Test Partial counterbalancing • Example: consider four conditions • Recall: ABCD = 4! = 24 possible orders 2) Balanced Latin square: each condition appears before and after all others

24. participants Colored words BW words Test Test 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.

25. Mixed factorial designs • Mixed 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). • This only works with factorial (multi-factor) designs • Next time: Factorial designs