The classic reference on this topic
is Baron, R. M., & Kenny, D. A. (1986). The moderator-mediator variable distinction
in social psychological research: Conceptual, strategic, and statistical considerations.
Journal of Personality and Social Psychology, 51, 1173-1182.
what is written here comes directly from this classic paper.
- Moderator variables
- "In general terms, a moderator is a qualitative (e.g., sex, race, class)
or quantitative (e.g., level of reward) variable that affects the direction
and/or strength of the relation between an independent or predictor variable
and a dependent or criterion variable. Specifically within a correlational
analysis framework, a moderator is a third variable that affects the zero-order
correlation between two other variables. ... In the more familiar analysis
of variance (ANOVA) terms, a basic moderator effect can be represented as
an interaction between a focal independent variable and a factor that specifies
the appropriate conditions for its operation." p. 1174
- Mediator variables -
"In general, a given variable may be said to function as a mediator to the
extent that it accounts for the relation between the predictor and the criterion.
Mediators explain how external physical events take on internal psychological
significance. Whereas moderator variables specify when certain effects will
hold, mediators speak to how or why such effects occur." p. 1176
general test for mediation is to examine the relation between the predictor
and the criterion variables, the relation between the predictor and the mediator
variables, and the relation between the mediator and criterion variables. All
of these correlations should be significant. The relation between predictor
and criterion should be reduced (to zero in the case of total mediation) after
controlling the relation between the mediator and criterion variables.
way to think about this issue is that a moderator
variable is one that influences the strength of a relationship between two other
variables, and a mediator
variable is one that explains the relationship between the two other variables.
As an example, let's consider the relation between social class (SES) and frequency
of breast self-exams (BSE). Age might be a moderator variable, in that the relation
between SES and BSE could be stronger for older women and less strong or nonexistent
for younger women. Education might be a mediator variable in that it explains
why there is a relation between SES and BSE. When you remove the effect of education,
the relation between SES and BSE disappears.
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updated 1 March 1999