Mediation:
The Indirect Effect
David A. Kenny
Mediation Webinars
• Basics
• Four Steps
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The Mediational Model
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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
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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).
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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%
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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.
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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.
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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.
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More Mediation
Webinars
• Testing the Indirect Effect
• Power and Effect Size
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