. Author manuscript; available in PMC: 2006 Jun 2.

Published in final edited form as: Control Clin Trials. 1999 Dec;20(6):573–600. doi: 10.1016/s0197-2456(99)00031-8

Table 3.

Power Projections for AMD Clinical Trial (Assumed placebo event rate = 0.17; n = 3060.)^a

	Rates of Progression to Advanced AMD^b				Approximate Power
Situation (see below)	Placebo(n = 765)	Antioxidants (n = 765)	Zinc (n = 765)	Antioxidants + zinc (n = 765)	α^c = 0.05	α^c = 0.01
1	0.162	0.128	0.128	0.094	0.87	0.68
2	0.163	0.109	0.109	0.109	0.84	0.65
3	0.170	0.170	0.116	0.170	0.84	0.65
4	0.164	0.164	0.117	0.117	0.86	0.67

Situation	Percent reduction	Due to
1 (Example: Main effects comparison: Antioxidants versus no antioxidants = 0.111 versus 0.145)	25	Antioxidants
	25	Zinc
	50	Antioxidants and zinc (no interaction)
2	40	Antioxidants
	40	Zinc
	40	Antioxidants and zinc (negative interaction)
3	0	Antioxidants
	40	Zinc
	0	Antioxidants and zinc (negative interaction)
4	0	Antioxidants
	35	Zinc
	35	Antioxidants and zinc (no interaction)

Calculated for 3600 participants with age-related macular changes; adjusted to 3060 for loss to followup and deaths prior to event.

Placebo rate of progression to advanced AMD of 0.17 is adjusted assuming 10% of participants would drop out and assume the placebo event rate and 10% of participants would drop in and assume the full treatment (antioxidants and zinc) event rate.

Two-sided α.