Table 3.

Estimated Changes in the GDR (Expressed in Percentage Points) as a Function of the Interventions and Copayments for Brand Name Drugs at Two Baseline GDR Rates (30 and 45 percent)

			PptChange at Baseline GDR of
Mode of Purchase	Intervention	Regression Coefficient	30%	45%	p-Value
Retail	Mailing	−0.248	−4.94	−6.03	< .0001
	Advertising	−0.006	−0.13	−0.15	.875
	Generic sampling	−0.001	−0.02	−0.02	.951
	Physician incentive	−0.016	−0.33	−0.40	.616
	Doubling copayments for brand name drugs	0.553	8.60	9.55	<.0001
Mail order	Mailing	−0.079	−1.63	−1.95	.232
	Advertising	−0.130	−2.65	−3.19	.297
	Generic sampling	−0.083	−1.71	−2.04	.301
	Physician incentive	−0.100	−2.06	−2.46	.406
	Doubling copayments for brand name drugs	0.19	2.84	3.27	.668

Each percentage-point change (PptChange) in the above table was derived using the formula where base is the baseline GDR, coefficient is the estimated regression coefficient of the intervention, δ is the change in the value of the intervention variable (δ=1 for the interventions; δ=log(2) for doubling the copayment ratio), and exp(.) denotes the exponential function. Note that this formula is just a mechanism for translating the parameter estimate to the probability scale; it does not compute the interaction or marginal effect that has been defined as in Ai and Norton (2003) but rather estimates the change in the probability that would have occurred had there not been an intervention.

GDR, generic dispensing rate