Mediation: The Indirect Effect

1. Mediation: The Indirect Effect David A. Kenny

2. Mediation Webinars • Basics • Four Steps

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

4. Decomposition of Effects Total Effect = Direct Effect + Indirect Effect c = c′ + ab Note that ab = c - c′ This equality exactly holds for multiple regression, but not necessarily for other estimation methods. Indirect Effect = ab Direct Effect = c′ Total Effect = c′ + ab

5. Percent of Total Effect Mediated 100[ab/c] or equivalently 100[1 - c′/c] This measure should only be computed if the total effect is statistically significant and moderate in size. It can be negative (when c′/c > 1) and greater than 100 (c′ have different signs).

6. Inconsistent Mediation • ab and c′ have a different sign • M as a “suppressor” variable • Example: Stress and Mood with Coping as a Mediator • Consequences • Step 1 may fail: If ab and c′ have a different sign, then the total effect or c will be small. • Percent mediated greater than 100%

7. Estimating the Total Effect (c) The total effect or c can be inferred from direct and indirect effect as c′ + ab. We need not perform Step 1 to estimate c. This can be useful in situations when c does not exactly equally c′ + ab.

8. All Four Steps Essential? Steps 1 and 4 are not so key. Steps 2 and 3 are essential. Note that ab measures the indirect or the amount of the total effect that is mediated. Need a way of testing the null hypothesis that the indirect effect of ab is zero. 8

9. Proximal vs. Distal Mediators Hoyle and Kenny define proximal mediation as the case in which a > b (effects standardized and positive) and distal mediation as b > a. Compliance is typically proximal and an intermediate outcome is distal. Power of the test of the indirect effect is much weak for proximal mediators. 9

10. More Mediation Webinars • Testing the Indirect Effect • Power and Effect Size