# General linear model with interaction term in SPSS

Following my question here, I will be most grateful for further assistance to clear my confusion.

The confusion is because I have read in many places that when you use ANCOVA in GLM (in SPSS), the effect of the covariate is controlled. I am unsure why it should be controlled in my case below (unless I am wrongly interpreting what a 'covariate' means). Or using the wrong test!

Question 1

What is the best method to analyse the following problem in SPSS?

I am looking at how the relationship between A and B behaves or differs based on gender. In other words, how does gender impact on the relationship between A and B. A and B are interval variables. A is the independent and B is the dependent variable. Gender, of course, is a categorical variable. (The nature of the variables, as I understand it, makes factorial ANOVA inappropriate, leading to a choice of ANCOVA for this example.)

My understanding is that B goes in the dependent variable box, gender goes in the factor box and A goes in the covariate box in SPSS GLM (univariate).

Question 2

*****What does the interaction mean?****

I understand I can get an interaction between A and gender, which then means that A and gender have a joint effect on B. If this is significant, does it mean that gender affects the relationship between A and B.

Question 3

Should I look at the effect of A and gender separately or only focus on the interaction between A and gender?

I am very new to GLM and it is fascinating (if I can get it right!).

• In addition to any answers below, I find Faraway's Practical Regression and ANOVA using R to be a concise and helpful reference for regression basics (available here: cran.r-project.org/other-docs.html). You can either go through the examples in R, or translate them to SPSS. I get the impression that your overarching question is, "How do I perform and interpret regression analyses". A good question, but one that most here studied in semester-long university courses, and some have spent careers studying. Nov 19, 2011 at 17:36