cb in the data set. It has got the levels
(0,0), (1,0), ..., (2,3) according to the respective dose
combinations. The data vectors with the different distributional
properties are ynrmhom, ynrmhet, yloghom and yloghet.
cb(0,0),
(0,1), ..., (2,3)ynrmhomynrmhetyloghomyloghet