I am using SPSS to run moderation analysis with categorical moderators. How to interpret the output with reference to the levels of the moderator categories?
@Mohua Gupta Interaction effect (Moderation) of a categorical variable on a IV ---> DV relationship either enhances or reduces the magnitude of an existing relationship with respect to changes in the moderator variable. For example, if Moderator is age; e.g. (1) 18-30 years, (2) 31-45 years, and (3) Above 45 years and interaction coefficient on IV --> DV relationship is .124 which means that when age increases by 1 SD (if interaction probe was set at -1 or +1 SD), IV --> DV relationship further strengthens up by 12.40%. Had you stated your study case, It would have been better for giving an explanation. Wishing you best for you research.
This is very well explained in Chapter 9 of Cohen, Cohen, West, and Aiken's (2003) regression text:
Cohen, J., Cohen, P., West, S. G. & Aiken, L. S. (2003). Applied multiple regression/correlation analysis for the behavioral sciences. Mahwah, NJ: Erlbaum.
Moderation is identical to testing for interaction effects, so you can create a set of dummy variables and multiply each of them times the independent variable that they are supposed to moderate.