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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.

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Assumptions

When you choose to run a moderator analysis using multiple regression, part of the process involves checking to make sure that the data you want to analyse can actually be analysed using multiple regression. You need to do this because it is only appropriate to use a moderator analysis using multiple regression if your data "passes" eight assumptions that are required for multiple regression to give you a valid result. In practice, checking for these eight assumptions just adds a little bit more time to your analysis, requiring you to click a few more buttons in SPSS Statistics when performing your analysis, as well as think a little bit more about your data, but it is not a difficult task.

Before we introduce you to these eight assumptions, do not be surprised if, when analysing your own data using SPSS Statistics, one or more of these assumptions is violated (i.e., not met). This is not uncommon when working with real-world data rather than textbook examples, which often only show you how to carry out a moderator analysis using multiple regression when everything goes well! However, don’t worry. Even when your data fails certain assumptions, there is often a solution to overcome this. First, let's take a look at these eight assumptions:

You can check assumptions #3, #4, #5, #6, #7 and #8 using SPSS Statistics. Assumptions #1 and #2 should be checked first, before moving onto assumptions #3, #4, #5, #6, #7 and #8. Just remember that if you do not run the statistical tests on these assumptions correctly, the results you get when running the moderator analysis might not be valid. This is why we dedicate a number of sections of our enhanced moderator analysis guide to help you get this right. You can find out about our enhanced content as a whole here, or more specifically, learn how we help with testing assumptions here. Alternately, you can access the enhanced moderator analysis guide now by subscribing to the site here.

In the section, Procedure, we illustrate the SPSS Statistics procedure to perform a moderator analysis assuming that no assumptions have been violated. First, we introduce the example that is used in this guide.

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Example

Cholesterol has a reputation for generally being bad and being a reason for getting heart disease. However, a particular type of cholesterol called high-density lipoprotein cholesterol (HDL cholesterol, for short) is linked to good heart health. The higher the concentration of HDL cholesterol in the blood the better.

It is known that exercise can increase HDL cholesterol concentration. However, there is thought to be a complex interplay between exercise and body fat. As such, although it is known that higher levels of physical activity are associated with higher concentrations of HDL cholesterol, a researcher wants to understand whether this relationship is similar in normal weight and obese individuals (as determined by their BMI [Body Mass Index], which is a way to assess whether individuals are of a normal weight or obese).

As such, the researcher hypothesized that individuals with higher levels of physical activity would have higher concentrations of HDL cholesterol, but that this relationship would be different for individuals who are normal weight and those who are obese. In variable terms, the researcher wants to know whether body_composition statistically significantly moderates the relationship between physical_activity and HDL.

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Setup in SPSS Statistics

In SPSS Statistics, we created three variables: (1) HDL, which is the HDL cholesterol concentration; (2) physical_activity, which is the participant's level of physical activity measured in the number of minutes of exercise performed per week; (3) body_composition, which is the participant's body composition (i.e., normal weight or obese). However, the moderator variable, body_composition, cannot simple be entered into a multiple regression equation. It first needs to be "converted" into a dummy variable. What this means and how to do it is explained in our enhanced moderator analysis guide. In this guide we name the dummy variable, normal. In addition, an interaction term has to be created between the independent and moderator variables, which we will call pa_x_normal (N.B., "pa" stands for the independent variable, "physical activity", "_x_" stands for multiplication and "normal" reflects our moderator variable). Again, why and how to do this is explained in our enhanced moderator analysis guide. You can learn about our enhanced data setup content here.

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Test Procedure in SPSS Statistics

The 11 steps below show you how to run a moderator analysis in SPSS Statistics when none of the eight assumptions in the previous section, Assumptions, have been violated. At the end of these 11 steps, we show you how to interpret the results from your moderator analysis. If you are looking for help to make sure your data meets assumptions #3, #4, #5, #6, #7 and #8, which are required when performing moderator analysis and can be tested using SPSS Statistics, you can access the enhanced moderator analysis guide by subscribing to the site here.

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Interpreting and Reporting the Output of the Moderator Analysis

SPSS Statistics will generate quite a few tables of output for a moderator analysis. In this section, we show you one of the tables you can use to determine whether body composition is moderating the relationship between physical activity and HDL cholesterol concentration, assuming that no assumptions have been violated. A complete explanation of the output you have to interpret when checking your data for the eight assumptions required to carry out a moderator analysis is provided in our enhanced guide. However, in this "quick start" guide, we focus only on one of the tables you can use to understand your moderator results, assuming that your data has already met these eight assumptions.

Therefore, to understand whether you have a moderator effect, you need to interpret the Model Summary table because this provides the change in R2 measure (within the "Change Statistics" columns for "Model 2"), which we can use to determine the statistical significance of the interaction term and, subsequently, whether body composition moderates the effect of physical activity on HDL cholesterol concentration, as highlighted below:

Published with written permission from SPSS Statistics, IBM Corporation.


The first column highlighted, "R Square Change", shows the increase in variation explained by the addition of the interaction term (i.e., the change in R2). You can see that the change in R2 is reported as .068, which is a proportion. More usually, this measure is reported as a percentage so we can say that the change in R2 is 6.8% (i.e., .068 x 100 = 6.8%), which is the percentage increase in the variation explained by the addition of the interaction term. We can also see that this increase is statistically significant (p < .0005), a result we obtain from the "Sig. F Change" column (remembering that, in SPSS Statistics, a statistical significance value of .000 does not mean zero, but p < .0005). We can conclude that body composition does moderate the relationship between physical activity and HDL cholesterol concentration.

The method we used above is one of two available to determine whether you have a statistically significant moderator effect. The other uses the statistical significance of the interaction term. This latter method also provides valuable information on the difference between the two groups of the moderator in their relationship between the independent and dependent variable. How to understand and interpret the interaction term is provided in our enhanced moderator analysis guide.

If we wanted to report the moderated multiple regression equation, we could do so by determining the coefficient values from the "B" column in the Coefficients table, as highlighted below:

Published with written permission from SPSS Statistics, IBM Corporation.

Using the values obtained above, you could report the regression equation as follows:

HDL = 32.694 + (0.016 x physical_activity) + (13.353 x normal) + (0.080 x pa_x_normal)

A fuller understanding of the equation above is provided in our enhanced moderator analysis guide.

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Follow up analyses

Once you have determined whether you have a statistically significant interaction, you can follow up with post hoc probing. One common approach is to consider the simple regression lines (aka simple regression slopes). The two simple regression slopes are shown in the diagram below:

Published with written permission from SPSS Statistics, IBM Corporation.

You can use follow up tests to determine whether these simple regression slopes are statistically significant. How to do this and interpret and report the results is presented in our enhanced moderator analysis guide. We also show you how to write up the results from your assumptions tests and moderator analysis output if you need to report this in a dissertation/thesis, assignment or research report. We do this using the Harvard and APA styles. You can view the enhanced moderator analysis guide by subscribing to the site here.

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References

A more extensive reference and bibliography is provided in the enhanced moderator guide.

Aguinis, H. (2004). Regression analysis for categorical moderators. New York, NY: Guilford Press.

Jaccard, J., & Turrisi, R. (2003). Interaction effects in multiple regression (2nd ed.). Thousand Oaks, CA: Sage Publications.

Jose, P. E. (2013). Doing statistical mediation & moderation. New York, NY: Guilford Press.

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