1. Experimental Design: Single factor designs Psych 231: Research Methods in Psychology

2. Announcements Reminder: your group project experiment Methods section (& Appendix) and IRB protocol are due in labs this week

3. Poorly designed experiments Bad design example 1: Does standing close to somebody cause them to move? “hmm… that’s an empirical question. Let’s see what happens if …” So you stand closely to people and see how long before they move Problem: no control group to establish the comparison group (this design is sometimes called “one-shot case study design”)

4. Single variable – One Factor designs 1 Factor (Independent variable), two levels Basically you want to compare two treatments (conditions) The statistics are pretty easy, a t-test T-test = Observed difference btwn conditions Difference expected by chance Although there are several types of t-tests Depends on your design

5. Poorly designed experiments Bad design example 2: Testing the effectiveness of a stop smoking relaxation program The subjects choose which group (relaxation or no program) to be in

6. Poorly designed experiments Non-equivalent control groups participants Training group No training (Control) group Measure Self Assignment Independent Variable Dependent Variable Random Assignment Problem: selection bias for the two groups, need to do random assignment to groups Problem: selection bias for the two groups, need to do random assignment to groups

7. Poorly designed experiments Bad design example 3: Does a relaxation program decrease the urge to smoke? Pretest desire level – give relaxation program – posttest desire to smoke

8. Poorly designed experiments One group pretest-posttest design participantsPre-test Training group Post-test Measure Independent Variable Dependent Variable Problems include: history, maturation, testing, and more Pre-test No Training group Post-test Measure Add another factor

9. 1 factor - 2 levels Good design example How does anxiety level affect test performance? Two groups take the same test Grp1 (moderate anxiety group): 5 min lecture on the importance of good grades for success Grp2 (low anxiety group): 5 min lecture on how good grades don’t matter, just trying is good enough

10. 1 factor - 2 levels participants Low Moderate Test Random Assignment Anxiety Dependent Variable

11. Single variable – one Factor anxiety low moderate 8060 lowmoderate test performance anxiety One factor Two levels Use a t-test to see if these points are statistically different

12. Single variable – one Factor Advantages: Simple, relatively easy to interpret the results Is the independent variable worth studying? If no effect, then usually don’t bother with a more complex design Sometimes two levels is all you need One theory predicts one pattern and another predicts a different pattern

13. Single variable – one Factor Disadvantages: “True” shape of the function is hard to see interpolation and extrapolation are not a good idea

14. Interpolation low moderate test performance anxiety What happens within of the ranges that you test?

15. Extrapolation lowmoderate test performance anxiety What happens outside of the ranges that you test? high

16. 1 Factor - multilevel experiments For more complex theories you will typically need more complex designs (more than two levels of one IV) 1 factor - more than two levels Basically you want to compare more than two conditions The statistics are a little more difficult, an ANOVA (Analysis of Variance)

17. Good design example (similar to earlier ex.) How does anxiety level affect test performance? Two groups take the same test Grp1 (moderate anxiety group): 5 min lecture on the importance of good grades for success Grp2 (low anxiety group): 5 min lecture on how good grades don’t matter, just trying is good enough 1 Factor - multilevel experiments Grp3 (high anxiety group): 5 min lecture on how the students must pass this test to pass the course

18. 1 factor - 3 levels participants Low Moderate Test Random Assignment Anxiety Dependent Variable High Test

19. 1 Factor - multilevel experiments anxiety low mod high 8060 lowmod test performance anxiety high

20. 1 Factor - multilevel experiments Advantages Gives a better picture of the relationship (function) Generally, the more levels you have, the less you have to worry about your range of the independent variable

21. Relationship between Anxiety and Performance lowmoderate test performance anxiety 2 levels highlowmod test performance anxiety 3 levels

22. 1 Factor - multilevel experiments Disadvantages Needs more resources (participants and/or stimuli) Requires more complex statistical analysis (analysis of variance and pair-wise comparisons)

23. Pair-wise comparisons The ANOVA just tells you that not all of the groups are equal. If this is your conclusion (you get a “significant ANOVA”) then you should do further tests to see where the differences are High vs. Low High vs. Moderate Low vs. Moderate

24. Next time Adding a wrinkle: between-groups versus within-groups factors Read chapter 11