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MacKinnon (2012) writes on p11 that

A moderator is a variable that changes the sign or strength of the effect of an independent variable on a dependent variable. It is typically (but not always) an interaction such that the effect of an independent variable depends on the level of the moderator variable.

As far as I can tell MacKinnon doesn't give an example of when this could happen. Presumably it's possible for a moderator to change the sign or strength of a an effect of the IV on the DV but for this to somehow not to depend on the level of the moderator.

Can anyone give a concrete example of when this could happen?

MacKinnon, D. P. (2012). Introduction to statistical mediation analysis. Routledge.

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The moderator effect, in practical terms, always corresponds to an interaction effect. The difference between the two is purely theoretical.

From "MacKinnon DP. Integrating Mediators and Moderators in Research Design. Research on social work practice. 2011;21(6):675-681":

"Moderator effects are also called interactions because the third variable interacts with the relation between two other variables. However, theoretically moderator effects differ slightly from interaction effects in that moderators refer to variables that alter an observed relation in the population while interaction effects refer to any situation in which the effect of one variable depends on the level of another variable."

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  • $\begingroup$ Thanks for the tip. I checked out the paper in question, but after reading it I'm still unsure of how a situation could arise in which the effect of one variable depends on the level of another variable, but this does not alter an observed relation in the population. Are you able to give an example of this? $\endgroup$ – user1205901 - Reinstate Monica Mar 10 '16 at 23:49
  • $\begingroup$ No, my guess from the paper I quoted is that in practical terms they are always the same, and that the distinction is purely theoretical. And this is the main reason why you seldom read about moderators in applied research. This, at least, in medical research. I am not aware of use of moderators in other fields, so I would be happy as well if someone else had a concrete example to show. $\endgroup$ – andbel Mar 11 '16 at 13:50

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