FAQ/power/propsn282013-03-08 10:17:39localhostconverted to 1.6 markup272012-05-03 14:36:38PeterWatson262012-05-03 13:12:17PeterWatson252012-05-03 13:12:09PeterWatson242010-05-11 15:14:07PeterWatson232010-05-11 14:48:35PeterWatson222010-05-11 14:48:08PeterWatson212010-01-28 15:10:07PeterWatson202009-12-01 10:25:37PeterWatson192007-09-06 16:10:27PeterWatson182007-09-06 16:10:10PeterWatson172007-09-04 15:17:40PeterWatson162007-03-20 16:08:04PeterWatson152007-01-15 09:54:28PeterWatson142007-01-15 09:53:56PeterWatson132007-01-11 17:01:53PeterWatson122007-01-11 17:01:40PeterWatson112007-01-11 16:22:11PeterWatson102007-01-11 14:25:01PeterWatson92007-01-11 14:24:50PeterWatson82007-01-11 14:22:38PeterWatson72006-10-24 10:55:44PeterWatson62006-10-23 15:58:39PeterWatson52006-10-23 15:54:43PeterWatson42006-10-23 15:54:08PeterWatson32006-09-15 10:50:53PeterWatson22006-09-15 10:50:08PeterWatson12006-09-15 10:48:25PeterWatsonThis program uses the difference in proportions (or equivalently the odds ratio) as the effect size. p1 and p2 are the proportions in groups 1 and 2 respectively being compared, delta is the ratio: size of group 2 / size of group 1, alpha is the (two-tailed) type I error. The program then outputs the sample sizes required for the specified power. A power calculator is also available in a spreadsheet or using a web calculator here. and in R and G*POWER3. An example uses marginal probabilities to compute cell probabilities in a 2x2 table to give inputs for a power analysis. [ADJUST THE EXAMPLE INPUT AS DESIRED; THEN COPY AND PASTE INTO A SPSS SYNTAX WINDOW AND RUN; OUTPUT BOTH TO SPREADSHEET AND OUTPUT WINDOW].