Hi Kathrin,
I agree with your first point.
In the second edition of the book, which I have in front of me (yours must be the 3rd ed.), the same example data are used at the end of chapter 6 ("Sets of s x r Tables", subsection 6.4.6). There, the authors only compute CMH statistics using PROC FREQ and do not apply PROC GLM.
So, now they have moved the example to chapter 7. Given its title, "Nonparametric Methods", I'm wondering why they apply PROC GLM to the original, untransformed data.
With SUBJECT being omitted from the CLASS statement, I think their code would be suitable for an ANCOVA model with the covariate SUBJECT. But SUBJECT is just a sequential number. Indeed, their results change if the subjects are numbered differently (e.g. the patient numbers 1 - 25 are randomly permuted). This can't be right.
The ESTIMATE statement seems less controversial to me. pH level has an interval scale. Therefore, if m1, m2, m3 and m4 are the parameter estimates corresponding to pH levels 6.5, 6.9, 7.4 and 7.9, it could make sense to calculate the "average pairwise difference as the pH level increases" as ((m4-m3)+(m3-m2)+(m2-m1))/3=(m4-m1)/3, which is what they do.
Best regards,
Reinhard
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