Path diagram:
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The authors clarify the meaning of mediation, with introducing a path diagram as a model for depicting a causal chain. The basic causal chain involved in mediation is diagramed in the figure above. "This model assumes a three-variable system such that there are two causal paths feeding into the outcome variable: the direct impact of the independent variable (Path c) and the impact of the mediator (Path b). There is also a path from the independent variable to the mediator (Path a).
A variable functions as a mediator when it meets the following conditions:
(a) variations in levels of the independent variable significantly account for the
variations in the presumed mediator (i.e., Path a),
(b) variations in the mediator significantly account for variations in the dependent
variable (i.e., Path b), and
(c) when Paths a and b are controlled, a previously significant relation between
the independent and dependent variable is no longer significant,
with the strongest demonstration of mediation occurring when Path c is zero.
In regard to the last condition we may envisage a continuum.
When Path c is reduced to zero, we have strong evidence for a single,
dominant mediator. If the residual Path c is not zero, this indicates the operation of
multiple mediating factors. Because most areas of psychology, including social, treat
phenomena that have multiple causes, a more realistic goal may be to seek mediators
that significantly decrease Path c rather than eliminating the relation between the
independent and the dependent variables altogether. From a theoretical perspective,
a significant reduction demonstrates that a given mediator is indeed potent, albeit
not both a necessary and a sufficient condition for an effect to occur
(Baron & Kenny, 1986, p. 1176).
See also: experiment, experimental design, moderator variable
Literature: Baron & Kenny (1986)
| Entry by: Eric Igou |
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June 11, 1999 Direct questions and comments to: Glossary master |
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