I have a dataset on two disorders, one measured on a continuous scale, and one with three categories (which can be collapsed into a yes/no variable if necessary). The two disorders have been measured at a maximum of 7 time points per individual, with irregular spacing in between.
I already know the two variables are correlated when you look at them cross-sectionally. But now I would like to know if there is a correlation in time, i.e., are changes in one condition associated with simultaneous changes in the other?
What would be the best way to analyse this? I read somewhere that it might be done with multiple regression, taking subject out as a factor, but with no further explanation, so I'm not sure this is indeed what I'm looking for. Any ideas?