Psyc 321_09 within groups

04/10/2023 CEDP321 Ryan Sain, Ph.D. 1

Developmental Research MethodsTopic 9: Within Groups Designs

Within groups designsAlso called repeated measuresWe are comparing groups of

people to themselvesOur own best controlsEach person in every group

receives all levels of the IV

AdvantagesAll groups are

equal at the beginning of the study

Fewer participantsVery sensitive to

changes (statistically)◦Within subject

variance is going to be low (error variance)

DisadvantagesCarryover effects

◦Seeing a picture vs. not seeing one◦Permanent change – cannot study in a

within subjects design◦Clinical treatments

Drug trials Psychotherapy

Sensitive to time variables (practice, fatigue, order of IV)◦Order effects and practice effects

Counterbalancing

Complete counterbalancing◦ Each condition

must occur with the same frequency

◦ Each condition must precede and follow all conditions an equal number of times

Controls for time and order effects

5!

Counterbalancing

Group 1 Treat 1 Treat 2 Treat 3

Group 3 Treat 1Treat 2 Treat 3

Group 2 Treat 1 Treat 2Treat 3

Group 4 Treat 1 Treat 2Treat 3

Group 5 Treat 1Treat 2 Treat 3

Group 6 Treat 1Treat 2Treat 3

More counterbalancing Incomplete counterbalancing

◦ Just balance the order.

Latin square – doing this randomly with the number of levels randomized for each participant

Differential order effects◦ certain orders may produce certain

outcomes – ◦ a sort of interaction = treatment x order

Group 1 Treat 1 Treat 2 Treat 3

Group 2 Treat 1Treat 2 Treat 3

Group 3 Treat 1Treat 2Treat 3

Mixed designs

Using between groups and within groups logic

Measures change over time within a group and compares a factor between groups as well◦2(between factor) x 3 (within factor

- often a time variable)

Mixed Designs cont.Rule out carryover effects

◦(using the between groups part) High levels of control of a within

design.Main effects

◦Time◦Other IV

Interactions◦Time x IV

Matched subjects designsReduce error varianceCombine the benefits of within subjects and

between groupsTwin studies – not always possibleMatch participants

◦ age, gender, SES, ethnicity, etc.◦ Then randomly assign one of each match to a

condition.

Matching - typesUse a matched group

as a control group◦ Random assignment

and equal groups.Use a matched group

as an experiment in unto itself◦ Two groups matched◦ Each group has both

males and females – treat as normal. Compare males treatment

to males control, etc

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◦Mixing table 1 - hovard◦Mixing table 2 - riverstorm◦People – biewoef◦Yin Yang - hisks◦Orchids – flajs01◦Drugs - alaasafei

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