I would like to do an ANCOVA on a set of 2 continuous data set (before and after pill) and one factor with 3 levels (3 pills) Here is the graph of before pill vs after pill with pill taken in colour:.
I would like to know how to evaluate the equality of regression slopes for each pill.
I was thinking of an ANOVA like test, which is for equality of means, but here for the slopes, that takes into account variance.
I saw this other option: Testing equality of coefficients from two different regressions, however I would be forced to make a pairwise analysis, and n the case where I have 1000 pills, might not be efficient.
Is there such a test? and how would it be implemented in r?