Experimental Control & Design Psych 231: Research Methods in Psychology

Controlling Variability Methods of Experimental Control Constancy/Randomization Comparison Production

Methods of Controlling Variability Constancy/Randomization If there is a variable that may be related to the DV that you can’t (or don’t want to) manipulate Control variable: hold it constant Random variable: let it vary randomly across all of the experimental conditions But beware of potential confounds, variables that co-vary with both the IV and DV but aren’t controlled

Methods of Controlling Variability Comparison An experiment always makes a comparison, so it must have at least two groups Sometimes there are control groups This is typically the absence of the treatment Training group No training (Control) group Without control groups if is harder to see what is really happening in the experiment It is easier to be swayed by plausibility or inappropriate comparisons

Methods of Controlling Variability Comparison An experiment always makes a comparison, so it must have at least two groups Sometimes there are control groups This is typically the absence of the treatment 1 week of Training group 2 weeks of Training group Sometimes there are a range of values of the IV 3 weeks of Training group

Methods of Controlling Variability Production The experimenter selects the specific values of the Independent Variables 1 week of Training group 2 weeks of Training group 3 weeks of Training group Need to do this carefully Suppose that you don’t find a difference in the DV across your different groups Is this because the IV and DV aren’t related? Or is it because your levels of IV weren’t different enough

Experimental designs So far we’ve covered a lot of the about details experiments generally Now let’s consider some specific experimental designs. Some bad designs Some good designs 1 Factor, two levels 1 Factor, multi-levels Factorial (more than 1 factor) Between & within factors

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”)

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

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 Bad design example 2:

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

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 Bad design example 3:

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 1 Factor (Independent variable), two levels Basically you want to compare two treatments (conditions) The statistics are pretty easy, a t-test

1 factor - 2 levels participants Low Moderate Test Random Assignment Anxiety Dependent Variable Good design example How does anxiety level affect test performance?

anxiety low moderate 8060 lowmoderate test performance anxiety One factor Two levels Use a t-test to see if these points are statistically different T-test = Observed difference between conditions Difference expected by chance Good design example How does anxiety level affect test performance? 1 factor - 2 levels

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 1 factor - 2 levels

low moderate test performance anxiety What happens within of the ranges that you test? Interpolation Disadvantages: “True” shape of the function is hard to see Interpolation and Extrapolation are not a good idea 1 factor - 2 levels

Extrapolation lowmoderate test performance anxiety What happens outside of the ranges that you test? Disadvantages: “True” shape of the function is hard to see Interpolation and Extrapolation are not a good idea 1 factor - 2 levels 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)

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

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