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