Moderator Analysis with a Dichotomous Moderator using SPSS Statistics

Introduction

A moderator analysis is used to determine whether the relationship between two variables depends on (is moderated by) the value of a third variable. This relationship is commonly between: (a) a continuous dependent variable and continuous independent variable, which is modified by a dichotomous moderator variable; (b) a continuous dependent variable and continuous independent variable, which is modified by a polytomous moderator variable; or (c) a continuous dependent variable and continuous independent variable, which is modified by a continuous moderator variable. In this guide, we focus on (a); namely, the relationship between a continuous dependent variable and continuous independent variable, which is modified by a dichotomous moderator variable.

We use the standard method of determining whether a moderating effect exists, which entails the addition of an (linear) interaction term in a multiple regression model. For this reason, you might often hear this type of analysis being referred to as a moderated multiple regression or as its abbreviation, MMR (e.g., Aguinis, 2004). Indeed, a moderator analysis is really just a multiple regression equation with an interaction term. What makes it a moderator analysis is the theory and subsequent hypotheses that surround this statistical test (e.g., Aguinis, 2004; Jaccard & Turrisi, 2003; Jose, 2013).

For example, a moderator analysis can be used to determine whether the relationship between HDL cholesterol and amount of exercise performed per week is different for normal weight and obese participants (i.e., the continuous dependent variable is "HDL cholesterol", the continuous independent variable is "amount of exercise performed per week" and the dichotomous moderator variable is "body composition", consisting of two groups: "normal weight" and "obese")? If it is, body composition (i.e., the

dichotomous moderator variable) moderates the relationship between the amount of exercise performed per week and HDL cholesterol concentration. Alternately, you could use a moderator analysis to determine whether the relationship between salary and years of education is moderated by gender (i.e., the continuous dependent variable is "salary", the continuous independent variable is "years of education" and the dichotomous moderator variable is "gender", which consists of two groups: "males" and "females"). If it is, gender (i.e., the dichotomous moderator variable) moderates the relationship between the years of education and salary.

This "quick start" guide shows you how to carry out a moderator analysis with a dichotomous moderator variable using SPSS Statistics, as well as interpret and report the results from this test. However, before we introduce you to this procedure, you need to understand the different assumptions that your data must meet in order for the moderator analysis to give you a valid result. We discuss these assumptions next.