Experimental Design: Single factor designs

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

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

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

11. One factor Use a t-test to see if these points are statistically different low moderate test performance low moderate anxiety Two levels Single variable – one Factor anxiety 60 80

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

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

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. 1 Factor - multilevel experiments • 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 • Grp3 (high anxiety group): 5 min lecture on how the students must pass this test to pass the course

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

19. anxiety mod high low test performance 60 80 low mod high anxiety 1 Factor - multilevel experiments 60

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. 2 levels 3 levels testperformance test performance low mod high low moderate anxiety anxiety Relationship between Anxiety and Performance

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