Experiment Basics: Designs Psych 231: Research Methods in Psychology

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 (but common) designs Some good designs 1 Factor, two levels 1 Factor, multi-levels Factorial (more than 1 factor) Between & within factors

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

1 factor - 3 levels participants Low Moderate Test Random Assignment Anxiety Dependent Variable High Test lowmod test performance anxiety high

Anxiety and Test Performance test performance highlowmod anxiety easy medium hard anxiety lowmodhigh main effect of difficulty 8060 main effect of anxiety Let’s add another variable: test difficulty. easy medium hard Test difficulty Interaction ? Yes: effect of anxiety depends on level of test difficulty

Factorial Designs Advantages Interaction effects –Always consider the interaction effects before trying to interpret the main effects – Adding factors decreases the variability –Because you’re controlling more of the variables that influence the dependent variable –This increases the statistical Power of the statistical tests – Increases generalizability of the results –Because you have a situation closer to the real world (where all sorts of variables are interacting)

Factorial Designs Disadvantages Experiments become very large, and unwieldy The statistical analyses get much more complex Interpretation of the results can get hard In particular for higher-order interactions Higher-order interactions (when you have more than two interactions, e.g., ABC).

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 (but common) designs Some good designs 1 Factor, two levels 1 Factor, multi-levels Factorial (more than 1 factor) Between & within factors

Example What is the effect of presenting words in color on memory for those words? Two different designs to examine this question Clock Chair Cab Clock Chair Cab Clock Chair Cab Clock Chair Cab So you present lists of words for recall either in color or in black-and-white.

participants Colored words BW words Test  2-levels  Each of the participants is in only one level of the IV Between-Groups Factor Clock Chair Cab Clock Chair Cab Clock Chair Cab Clock Chair Cab levels

participants Colored words BW words Test  2-levels, All of the participants are in both levels of the IV Clock Chair Cab Clock Chair Cab Clock Chair Cab Clock Chair Cab levels  Sometimes called “repeated measures” design Within-Groups Factor

Between vs. Within Subjects Designs Within-subjects designs All participants participate in all of the conditions of the experiment. participants Colored words BW words Test participants Colored words BW words Test 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. participants Colored words BW words Test participants Colored words BW words Test Between-subjects designs Each participant participates in one and only one condition of the experiment. Between vs. Within Subjects Designs

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 Sometimes this is a ‘must,’ because you can’t reverse the effects of prior exposure to other levels of the IV No order effects to worry about Counterbalancing is not required participants Colored words BW words Test Clock Chair Cab Clock Chair Cab Clock Chair Cab Clock Chair Cab

Between subjects designs Disadvantages Individual differences between the people in the groups Excessive variability Non-Equivalent groups participants Colored words BW words Test Clock Chair Cab Clock Chair Cab Clock Chair Cab Clock Chair Cab

Individual differences The groups are composed of different individuals participants Colored words BW words Test

Individual differences The groups are composed of different individuals participants Colored words BW words Test Excessive variability due to individual differences Harder to detect the effect of the IV if there is one R NR R

Individual differences The groups are composed of different individuals participants Colored words BW words Test Non-Equivalent groups (possible confound) The groups may differ not only because of the IV, but also because the groups are composed of different individuals

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

Matching groups Group AGroup 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 Brown tall 22yrs Color Height Age matched Red Short 21yrs matched Blue tall 23yrs matched Green average 22yrs matched Brown tall 22yrs

Within-subjects designs All participants participate in all of the conditions of the experiment. participants Colored words BW words Test participants Colored words BW words Test Between-subjects designs Each participant participates in one and only one condition of the experiment. Between vs. Within Subjects Designs

Within subjects designs Advantages: Don’t have to worry about individual differences Same people in all the conditions Variability between conditions is smaller (statistical advantage) Fewer participants are required

Within subjects designs Disadvantages Range effects Order effects: Carry-over effects Progressive error Counterbalancing is probably necessary to address these order effects

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.

test Condition 2Condition 1 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?

Order effects Progressive error Practice effects – improvement due to repeated practice Fatigue effects – performance deteriorates as participants get bored, tired, distracted

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

Counterbalancing Simple case Two conditions A & B Two counterbalanced orders: AB BA participants Colored words BW words Test Colored words BW words Test Note: this becomes a factorial design

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

Partial counterbalancing Example: consider four conditions Recall: ABCD = 4! = 24 possible orders 1) Unbalanced Latin square: each condition appears in each position (4 orders) DCBA ADCB BADC CBAD Order 1 Order 2 Order 3 Order 4

Partial counterbalancing 2) Balanced Latin square: each condition appears before and after all others (8 orders) ABDC BCAD CDBA DACB ABCD BCDA CDAB DABC Example: consider four conditions Recall: ABCD = 4! = 24 possible orders

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

Describing your design You need to describe: How many factors How many levels of each factor Whether the factors are within or between groups e.g., 2 (shallow/deep processing) x 2 (abstract/concrete) mixed groups factorial design

Describing your results You need to report: The main effects Depth of processing Word Type The interaction For each report the means (in the case of the main effects, report the marginal means) and the statistical outcomes (the ANOVA results) Depth of processing: F(1,226) = 98.6, p < Word type: F(1,226) = 34.0, p < Interaction: F(1,226) = 5.0, p < Do this with within complete sentences and paragraphs Feel free to supplement the text with a graph if it helps with clarity. abstractconcrete Shallow Deep