I have 31 observations for 9 subjects undergoing therapy (4 session for 4 subjects, and 3 session for the rest). I have implemented a mixed model to assess relationships between the observations using Fisher-transformed correlation coefficients.
For example, to assess the effect of finger tapping on correct responses, I correlated the number of finger taps with the percentage of correct responses for each subject (3 pairs of observations for some subjects, and 4 for others). I then perform a Fisher-transform on the 9 resulting correlation coefficients to get 9 z-scores. Then it's a simple matter of testing whether or not those z-scores differ from 0.
What I would like to do is control for subjects' age. However, I'm not clear on the best way to do this. It's easy to calculate the covariance between the z-scores and subject age, but how do I get the effect of finger taps on correct responses, controlling for age?
(As a side note, I've implemented this as a MIXED model in SPSS, but it fails to converge, so I can't trust the results).