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Methodology
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Assessment of interaction

28/07/2012 08:36' PM

Introduction

Often the relationship between two variables is modified by another control variable. This means that the strength and/or direction of an association varies according to the value of one or more additional variable(s).

Definition:

Interaction exists when the true association between two variables changes as the value of a third variable changes.[1]

The interaction effect is multiplicative as demonstrated in the figure below. The independent variables, X and Z, have an effect on outcome, Y. In addition, the combined variable, XZ, also has an effect on Y. This means that removing Z will change the effect of X on Y and vice versa.

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