Experimental Design: Between and within factors Psych 231: Research Methods in Psychology

Announcements Exam 2 coming up (Nov. 1) In labs –Turn in methods sections for group projects –Turn in IRB form (in PIP packet) –Be prepared to pilot your studies next week

Between versus Within Subjects Designs 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.

Example What is the effect of presenting words in color on memory for those words? –So you present lists of words for recall either in color or in black-and-white.

1 Between factor - 2 levels participants Colored words BW words Test  So each of the participants is in only one level of the IV levels Clock Chair Cab Clock Chair Cab

1 Within factor - 2 levels participants Colored words BW words Test  So all of the participants are in both levels of the IV levels Club Chain Cat Clock Chair Cab

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 No order effects to worry about –Counterbalancing is not required Sometimes this is a ‘must,’ because you can’t reverse the effects of prior exposure to other levels of the IV

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

Individual differences Non- Equivalent groups –The groups may differ not only because of the IV, but also because the groups are composed of different individuals

Dealing with Individual Differences 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 Matching groups - match each individuals in one group to an individual in the other group on relevant characteristics

Matching groups Group AGroup B matched

Individual differences Excessive variability due to individual differences –Harder to detect the effect of the IV if there is one R NR exp NR other NR other R

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

Within subjects designs Disadvantages –Order effects: Carry-over effects Progressive error Counterbalancing is probably necessary –Range effects

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.

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

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 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 ABDC BCAD CDBA DACB ABCD BCDA CDAB DABC

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.

Mixed factorial designs Mixed designs –This only works with factorial (multi-factor) 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).