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Basics of Mediation

Basics of Mediation. David A. Kenny. Interest in Mediation. Mentions of “mediation” or “mediator” in psychology abstracts: 1980: 36 1990: 122 2000: 339 2010: 1,198. 2. Why All the Interest in Mediation?. Fundamental reason: Mediation is one way to answer the question of “How?”

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Basics of Mediation

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  1. Basics of Mediation David A. Kenny

  2. Interest in Mediation Mentions of “mediation” or “mediator” in psychology abstracts: 1980: 36 1990: 122 2000: 339 2010: 1,198 2

  3. Why All the Interest in Mediation? • Fundamental reason: Mediation is one way to answer the question of “How?” • I have an effect, and I need understand the process through which it works.

  4. The Mediational Model Y X

  5. The Mediational Model c' Y X

  6. The Mediational Model M Y X

  7. The Mediational Model M Y X

  8. The Mediational Model M b a Y X

  9. The Mediational Model M b a Y X

  10. The Mediational Model M b a c' Y X

  11. Mediation versus Moderation • Mediation postulates a causal chain • X  M  Y • Moderation postulates that a causal effect changes as a function of another variable • The X  Y varies for different values of M.

  12. Different Reasons for Interest in Mediation • Theory elaboration • Program design • Understand why the intervention did not work • Find more proximal endpoints • Causal modeling • Power considerations

  13. Theory Elaboration • Researchers start an effect, e.g., mere exposure (brief exposures lead to liking). • What to know what is the process and they propose a mediator, e.g., cognitions. • Theory advances, e.g., cognitions do not appear to mediate the mere exposure effect and subliminal mechanisms were specified.

  14. Program Design • Dave MacKinnon notes that prevention researchers start with a outcome: adolescent smoking. • They use science (developmental psychology & sociology) to determine what causes the outcome (peer pressure) which becomes the mediator. • Then then design an intervention to trigger the mediator.

  15. Understanding Why a Program Did Not Work M b a Y X

  16. Find More Proximal Endpoints • Sometimes it takes a long-time to measures a key outcome: • Disease onset • Mortality • Divorce • By finding a proximal endpoint, research turnaround can be quicker. • Cholesterol levels can be used instead of heart disease.

  17. Theory of Planned Behavior • Fishbein & Ajzen • Attitudes & Social Norms  Intention  Behavior • Because “Intention” is a mediator, it can be used as a proxy for behavior change.

  18. Causal Modeling • Tests of structural equation models (i.e., over-identified models) involve missing paths. • Two different ways a path between X and Y can be missing: • Mediation: X  M  Y • Spuriousness: Z  X and Z  Y. • Mediation is much more interest and so typically the missing path is a mediation path not a “spurious” path.

  19. Power Considerations • Often the key part of a causal model is the mediational piece. • Tests of a causal model are either due to mediation or due to spuriousness. • Mediation is much more theoretically interesting than spuriousness. • Understand why the intervention did not work • Find more proximal endpoints • Tests of mediation relatively powerful

  20. Level of Measurement • M and Y are measured at the interval level of measurement. • X need not be and may be a dichotomy

  21. Mediation Webinars • Early History • Four Steps • Indirect Effect • Testing the Indirect Effect • Detailed Example

  22. More Webinars • Assumptions • Solutions to the Violations of Assumptions • Sensitivity Analyses • Power and Effect Size • PowMedR • Causal Inference Approach

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