. Author manuscript; available in PMC: 2008 Oct 6.

Published in final edited form as: Ophthalmology. 2007 Feb;114(2):271–277. doi: 10.1016/j.ophtha.2006.09.016

Table 6.

Sample Size Calculations

		2-yr Follow-up	1-yr Follow-up
Type of Treatment	Outcome Variable	Total Patients Required	Total Patients Required
Systemic treatment, without prior data	Difference in ER between groups	306 (153 per group)^*	378 (189 per group)^†
Systemic treatment, knowing prior enlargement rate	Difference in ER (ER minus prior ER) between groups	220 (110 per group)^*	Not applicable
Uniocular treatment, using fellow eye as control	Difference in ER between eyes	38	103

ER = enlargement rate.

α = 0.05, power = 0.80 for all calculations.

Difference to detect is a 25% reduction (0.64 mm²/yr) in enlargement rate.

Assuming 15% loss to follow-up for 2-year estimates, and 10% loss to follow-up for 1-year estimates.

Sample size is based on distribution for 1 eye per patient.

For systemic treatment, half the patients would be in treatment group and half in control group.

^†

The 1-year enlargement rate data included only those patients for whom he first follow-up visit was between 9 and 15 months after baseline.