Psyc 321_09 within groups
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Transcript of Psyc 321_09 within groups
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04/10/2023 CEDP321 Ryan Sain, Ph.D. 1
Developmental Research MethodsTopic 9: Within Groups Designs
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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
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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)
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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
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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!
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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
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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
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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)
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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
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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.
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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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