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

Announcements Reminder: your group project experiment method section is due in labs this week

Experimental designs So far we’ve covered a lot of the details of experiments, now let’s consider some specific experimental designs. –1 Factor, two levels –1 Factor, multi-levels –Factorial (more than 1 factor) (next Monday’s lecture) –Between & within factors (this Wednesday’s lecture)

Single variable – One Factor designs 1 Factor, 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

1 factor - 2 levels 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

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

Single variable – one Factor anxiety low moderate 8060 lowmoderate test performance anxiety One factor Two levels

Single variable – one Factor Advantages: –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

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

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

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

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)

1 Factor - multilevel experiments Example (same as before with one more group) –How does anxiety level affect test performance? Three 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

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

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

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

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

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

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

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