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Chapter 11 Human Participants Review. Wednesday, July 16. Single-variable, Correlated-Groups Designs. Introduces a correlation between groups in the way groups are formed Within-subjects design: Same participants in each group Matched-groups design Groups formed by matched random assignment.

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Chapter 11 Human Participants Review

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Chapter 11 human participants review l.jpg

Chapter 11Human Participants Review

Wednesday, July 16


Single variable correlated groups designs l.jpg

Single-variable, Correlated-Groups Designs

  • Introduces a correlation between groups in the way groups are formed

    • Within-subjects design:

      • Same participants in each group

    • Matched-groups design

      • Groups formed by matched random assignment


Correlated groups designs l.jpg

Correlated-Groups Designs

  • More sensitive than independent-groups designs

    • Controlling for individual differences makes it easier to detect small effects

  • The existence of a correlation between conditions has important implications for design and analysis


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Within-Subjects Designs

  • All participants are exposed to all experimental conditions

  • Each participant serves as “his or her own control”

  • In this way individual differences are removed from the treatment effect


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Within-Subjects Designs

  • Need to control for sequence effects

    • Sequence effects result from the experience with one condition affecting the performance in subsequent conditions

    • Controlled by varying the order of presentation (such as with counterbalancing)


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Statistical Analysis

  • Appropriate Statistical Analyses

    • Correlated t-test (for 2 groups only)

    • Repeated measures ANOVA

  • Order data so that each line represents one participant and each row represents one condition

    • Note that the columns represent conditions, NOT the order of testing


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Within-Subjects Strengths

  • More sensitive to small group differences because the variability due to individual differences is statistically eliminated

  • Fewer participants are needed because each participant appears in each condition

  • Instructions may take less time because participants were already instructed on the task in previous conditions


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Within-Subjects Weaknesses

  • Because participants experience all conditions, they may figure out the hypothesis (potential subject effects)

  • Major issue is sequence effects

    • Practice and carry-over effects

    • Controlled by varying the order of presentation

      • Counterbalancing

      • Random order of presentation

      • Latin square design


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Matched-Subjects Designs

  • Introduces correlation by matching the participants in each group with participants from the other groups

  • Should match on “relevant” variables

    • Variables that affect the dependent variable

    • Variables that show considerable natural variation in the population sampled


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Matching Participants

  • Match participants in sets, where the size of the set is equal to the number of conditions

  • Matching gets more difficult as:

    • The number of matching variables increases

    • Matching is done on continuous variables

    • The number of conditions increase

  • Once sets are matched, you randomly assign the participants in the set to the conditions


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Statistical Analysis

  • Analyze as if it were a within-subjects study

    • Data from matched participants are organized as if the data came from a single participant

  • Tell the program that the number of participants was equal to the actual number of participants divided by the number of conditions

    • e.g., for 40 participants and 4 conditions, tell the program that you had 10 participants and 4 conditions in a within-subjects design


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Strengths and Weaknesses

  • Strengths

    • Increased sensitivity to small differences between groups,but without the sequence effects of within-subjects designs

  • Weaknesses

    • Extra work of matching participants

    • Participants without appropriate matches cannot be used in the study


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Summary

  • Can introduce a correlation in two ways

    • Within-subjects designs

    • Matched-subjects designs

  • These designs are more sensitive to small differences between groups

  • The costs for the greater sensitivity are:

    • Sequence effects (within-subjects design)

    • Matching difficulties (matched-subjects design)


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