Psychology 513: Advanced Research Methods

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Psychology 513 Advanced Research Methods

• Moderators and Mediators

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Moderators (i.e., interactions)

• Relationship between IV and DV changes at the
  level of a third variable
• Specifies the conditions under which a
  relationship holds or is stronger/weaker
• Example procedural justice distributive
  justice on attitudes

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Moderated Multiple Regression (MMR)

• X predictor, Z moderator, Y criterion
• Yhat a b1X b2Z b3XZ
• Interaction term is represented by the
  multiplication of X by Z
• Test of the interaction is the test of b3

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Centering in MMR

• MMR includes main effects and interaction, which
  usually results in high multicollinearity
• Reduce multicollinearity by centering variables
  involved in interaction
• Simply subtract out the mean of the variable

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Example

• Y self-assurance
• X time in position Z managerial ability
• Yhat 2.54 1.14X 3.58Z 2.58XZ
• The b weights for Z and XZ are significant

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Interpretation

• When centered, b1 and b2 weights represent the
  effects of the predictors at the mean of the
  other predictors
• b3 represents the angle between the regression
  lines of Y on X at values of Z

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Graphing the Interaction
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Overview of Mediators

• (Baron Kenny, 1986)
• Attempts to answer why or how an effect occurs
• Explain why the IV affects the DV
• Example weapons effect aggression

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Mediators

• Mediator
• Independent Outcome
• Variable Variable

B
A
C
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Must meet the following

• IV is related to mediator (Path A)
• Mediator is related to DV (Path B)
• When Paths A B are controlled, Path C becomes
  non-significant (partial mediation) or zero
  (complete mediation)

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Testing Mediation

• Regress mediator on IV
• Regress DV on IV
• Regress DV on IV and mediator

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Example Realistic Recruitment

• IV favorability of message (positive-only vs.
  realistic)
• DV attraction
• Mediators perceived challenge

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Test of Mediator

• Regressed challenge on favorability of message (b
  0.27 t(223) 2.09, p .04)
• Regressed attraction on favorability of message
  (b 0.29 t(223) 2.03, p .04)
• Regressed attraction on favorability challenge
  (see next slide)

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Test of Mediator (cont.)