There is often confusion among students concerning the difference between a mediator variable and a moderator variable. The explanation involves some concepts that haven't been introduced at this point but it can still be worthwhile to discuss these types of variables.

In general, a given variable may be said to function as a mediator to the extend 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.

Mediator Variable

Researchers clarify the meaning of mediation, by introducing path diagrams as a models for depicting a causal chain. The basic causal chain involved in mediation is diagramed in the Figures 1 & 2 above. These models assumes a three-variable system such that there are two paths feeding into the outcome variable: the direct impact of the IV on the DV and the indirect path from the IV to the DV via the MV. There is also a relation between the MV and the DV.

A variable functions as a mediator when it meets the following conditions:
(a) variations in levels of the IV significantly account for the variations in the presumed mediator (Figure 1),
(b) variations in the MV significantly account for variations in the DV, and
(c) when both IV and MV appear in the model, a previously significant relation between the IV and DV is no longer significant, with the strongest demonstration of mediation occurring when the direct IV to DV path is zero (Figure 2).

Moderator Variable

In analysis of variance (ANOVA) terms, a moderator effect can be represented as an interaction between a major independent variable and a factor that specifies the appropriate conditions for its operation, that is, the effect of the major independent variable depends upon the value of the moderator variable. Consider, for example, a research study looking at two different methods of teach mathematics. If students with strong reading skills do better with one method and those with low reading skills do better with the other than reading is functioning as a mederator variable.

Example:

	method 1	method 2
hi achiev	20	50
lo achiev	40	30

References:

Baron, R. & Kenny, D. (1986) The moderator-mediator variable distinction in social psychological research: Conceptual, strategic, and statistical considerations, Journal of Personality and Social Psychology, 51, 1173-1182.

Stern, G.S.; McCants, T.R. & Pettine, P.W. (1982) The relative contribution of controllable and uncontrollable life events to stress and illness. Personality and Social Psychology Bulletin, 8, 140-145.

Adapted from SFB 504 Glossary

Intro Home Page