I'm not entirely sure this would be the correct place to ask, but Im in the process of analyzing data for my thesis and realized that I need to conduct an ANCOVA on my data. While I've taken a number of statistics courses, I have never come across ANCOVAs and I wanting to get some feedback on the correct way to conduct this analysis.

I have 3 2level factors (2x2x2):

var1 - within subjects

var2 - within subjects

var3 - between subjects

and a covariate that is continuous

Im expecting to find a significant 3 way interaction once the effect of the covariate has been removed, and then be able to assess the 2 2ways within the same analysis

In SPSS, I can go to analyze > GLM > repeated measures, add all the within, between variables and covariate

Onto my questions:

1. In the output, I assume I look at var1*var2*var3 to assess my 3 way interaction? Is this with the effect of the covariate removed?

2. In the same output, what does var1*var2*covariate tell me? Is that relevant to my question?

3. Assuming I look at the var1*var2*var3 row, I do in fact get a significant 3 way interaction. Now I need to assess whether there are significant 2 way interactions between var1*var2 at the different levels of var3. I understand that there are going to be alpha issues if I just split the file by var3 and run some 2 way ANCOVAS - what is the correct way to conduct these analyses, ideally at the same time as conducting the initial 3 way? Can SPSS give me all that output in one go?

4. How do I then go on to assess the simple main effects of those 2 way interactions?

5. Are there assumptions that ANCOVA makes that I should be testing prior to doing any of this? And how are those assumptions tested in SPSS?

Basically, Im trying to get a bunch of information: 3 way, 2 ways, simple main effects, main effects, all in a single analysis without having to do any file splitting and without inflating/deflating my alpha level

thanks!