I ran a cross-over trial where healthy volunteers came to the lab twice and underwent sensory testing at both arm and foot, before and after one of two neuromodulation interventions. Each participant underwent both conditions in a randomly assigned order. We want to see if the neuromodulation interventions affected the outcomes differently for the arm and the foot.
I would like to analyze the (continuous) outcomes with a mixed model. My fixed effects are Time (baseline and poststim), Stimulation (two levels) and Limb (arm and foot). I added Subject as a Random effect.
My problem is the specification of the Repeated Structure. The default is set to Residual. But, as described in the JMP manual: "The Residual structure specifies that there is no covariance between observations, namely, the errors are independent." This is obviously not the case here.
When I try more appropriate structures, I get into trouble. Because I have four values for each time point for each subject (e.g., timepoint baseline: for each limb and for each stimulation), I get error messages like the one below:
The XXX covariance model requires that no subjects have duplicate values of the Repeated input variable.
Any help that can point me to the best specification for this model would be greatly appreciated.
