Mediation The Indirect Effect David A Kenny Mediation

Mediation: The Indirect Effect David A. Kenny

Mediation Webinars • Basics • Four Steps 2

The Mediational Model 3

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 4

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). 5

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% 6

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. 7

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

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

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