Wednesday, April 20, 2011

Sample Size for Proportional Odds Models

Over the summer we needed to determine the sample size for a SNP-related grant where response is 0/1/2. Turns out Hmisc package in R has a function "popower". Here is the descripton:

"popower computes the power for a two-tailed two sample comparison of ordinal outcomes under the proportional odds ordinal logistic model. The power is the same as that of the Wilcoxon test but with ties handled properly. posamsize computes the total sample size needed to achieve a given power. Both functions compute the efficiency of the design compared with a design in which the response variable is continuous. print methods exist for both functions. Any of the input arguments may be vectors, in which case a vector of powers or sample sizes is returned. These functions use the methods of Whitehead (1993)."

Whitehead J (1993): Sample size calculations for ordered categorical data. Stat in Med 12:2257–
2271.

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