Solved – How to do repeated measures ANOVA for two groups in SPSS and 12 time points

anovarepeated measuresspss

I am measuring one parameter (say weight) repeatedly at different time points (say monthly, for 1 year) for two groups (say experimental and control groups). My data looks something like data posted in this question: Analyzing repeated measures ANOVA with two groups. I want to compare the change in weight over time 'between' the two groups. I think I should use repeated measures ANOVA for this. In SPSS, when I try do that I'm able to get 'within' group comparison of the parameter over time but not 'between' group.

Can somebody help?

Best Answer

In order to get the between factors box, you must have a column for "groups" and the remaining columns of time up to a year. With this said, each row will be giving information for a subject.

This testing comes under mixed ANOVA where you have repeated measures as well as between groups.

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Solved – Analyzing repeated measures ANOVA with two groups

Even for one group only, I don't like the so-called repeated measures ANOVA because it assigns a compound symmetry variance matrix, thereby implying the same correlation between week1 and week2 and between week1 and week3, which is a contestable assumption.

But in fact I'm not sure that all statisticians are in agreement in regards to the meaning of "repeated measures ANOVA". It is more important to understand what we do than only knowing the name of what we do.

The easiest way is to assign an unstructured variance matrix, and this is the so-called MANOVA (by the way, here I was asking about an intermediate between compound symmetry and unstructured).

In the SAS language, your model would be

PROC MIXED DATA=mydata;
MODEL y=group;
REPEATED week / SUBJECT=subject TYPE= *set your type here*;

Setting TYPE=UN is the MANOVA with the group factor. Setting TYPE=CS is the "classical" repeated-measures ANOVA with the group factor.

Solved – Repeated measures ANOVA for two different groups

You could do a repeated measures ANOVA (over time, e.g. pre/1 mo/2 mo) for each of your DVs separately. I don't have a copy of SPSS at the present time, but this primer looks about right.

If your dependent measures are moderately correlated (rs = .4 to .7), you may want to consider looking into a MANOVA or creating some composite measures. Unfortunately, I do not know much about how to conduct a MANOVA inside SPSS. So, I leave that to better minds.

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Solved – Analyzing repeated measures ANOVA with two groups

Solved – Repeated measures ANOVA for two different groups

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