In change analyses (of any variable, from pre to post) we usually controll for the pre-score, as it may affect the rate of change. However, what if I am especially interested in the effect of the pre score on the rate of change, like
Diff(post - pre) = a + b*pre.
What if I am especially interested in b? Is the model above reasonable or are theirthere any problems with that?
In multilevel-analysis of longitudinal data there is a correlation coefficient for the relation between the initial level (intercept) and the rate of change (slope). Thats the information I need, but in an ordinary regression model.