Solved – Analyzing repeated measures ANOVA with two groups

anovarepeated measures

I have a data set that looks like this:

GROUP                    WEEK1 WEIGHT(kg)          WEEK2 WEIGHT(kg)         WEEK3 WEIGHT(kg)
Control
  Subject C1             40                        42                       44
  Subject C2             52                        57                       57 
  Subject C3             55                        55                       60
  Subject C4             45                        47                       49

Experimental
  Subject E1             38                        40                       49
  Subject E2             42                        47                       50 
  Subject E3             55                        60                       68
  Subject E4             65                        77                       88

My question is: What tool shall I use to test my hypothesis that the two groups (control and experimental) do not significantly differ with regard to weight?

Please note that the weights are repeated measures. I understand that if I were to examine the data per group (Control first, then Experimental), I'd perform a repeated measures ANOVA for control and another for experimental.

Best Answer

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.

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