D1	=	1 if DAC > 0 (substitution in favor of less expensive drug)
	=	0 otherwise
D2	=	1 if DAC < 0 (substitution in favor of more expensive drug)
	=	0 otherwise
D3	=	1 for independent pharmacy
	=	0 for chain
i	=	drug (as defined earlier)
j	=	type of pharmacy (chain or independent)
k	=	observation
E(uijk)2 = σij2 and E(uijkuijk) = 0