Basics of Mediation
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Basics of Mediation David A. Kenny
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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?” - PowerPoint PPT Presentation
Transcript of Basics of Mediation
Basics of MediationDavid A. Kenny
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Interest in Mediation
• Mentions of “mediation” or “mediator” in psychology abstracts:
– 1980: 36
– 1990: 122
– 2000: 339
– 2010: 1,198
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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.
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The Mediational Model
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The Mediational Model
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The Mediational Model
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The Mediational Model
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The Mediational Model
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The Mediational Model
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The Mediational Model
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Mediation versus Moderation
• Mediation postulates a causal chainX M Y
• Moderation postulates that a causal effect changes as a function of another variableThe X Y varies for different
values of M.
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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
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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.
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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.
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Understanding Why a Program Did Not Work
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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.
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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.
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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.
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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
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Level of Measurement
• M and Y are measured at the interval level of measurement.
• X need not be and may be a dichotomy
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Mediation Webinars• Early History• Four Steps• Indirect Effect• Testing the Indirect Effect• Detailed Example
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More Webinars• Assumptions• Solutions to the Violations of
Assumptions• Sensitivity Analyses• Power and Effect Size• PowMedR• Causal Inference Approach
