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

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10 The Mediational Model

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

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