Spotlights, Floodlights, and the Magic Number Zero: Simple Effects Tests in Moderated Regression

Abstract

It is common for authors discovering a significant interaction of a measured variable X with a manipulated variable Z to examine simple effects of Z at different levels of X. These “spotlight” tests are often misunderstood even in the simplest cases, and it appears that consumer researchers are unsure how to extend them to more complex designs. We explain the general principles of spotlight tests, show that they rely on familiar regression techniques, and provide a tutorial showing how to apply these tests across an array of experimental designs. Rather than following the common practice of reporting spotlight tests at one standard deviation above and below the mean of X, we recommend that when X has focal values, researchers report spotlight tests at those focal values. When X does not have focal values, we recommend researchers report ranges of significance using a version of Johnson and Neyman’s (1936) test we call a “floodlight”.

Keywords: moderated regression, spotlight analysis, simple effects tests