power.t.test() requires a standard deviation as input, but for a two sample test, I do not understand what is required. Is it the standard deviation of the means (standard error), or of the combined sample? thanks, Hank Stevens Martin Henry H. Stevens, Assistant Professor 338 Pearson Hall Botany Department Miami University Oxford, OH 45056 Office: (513) 529-4206 Lab: (513) 529-4262 FAX: (513) 529-4243 http://www.cas.muohio.edu/botany/bot/henry.html http://www.muohio.edu/ecology
>-----Original Message----- >From: r-help-bounces at stat.math.ethz.ch >[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Hank Stevens >Sent: Monday, March 31, 2003 2:43 PM >To: r-help at stat.math.ethz.ch >Subject: [R] power.t.test > > >power.t.test() requires a standard deviation as input, but for >a two sample >test, I do not understand what is required. Is it the standard >deviation of >the means (standard error), or of the combined sample? > >thanks, >Hank StevensThe ''sd'' argument is the standard deviation of the mean for *each* sample, keeping in mind that the underlying assumption is that both samples have equal variances. Thus you need only specify one value. HTH, Marc Schwartz
Hank Stevens <HStevens at muohio.edu> writes:> power.t.test() requires a standard deviation as input, but for a two > sample test, I do not understand what is required. Is it the standard > deviation of the means (standard error), or of the combined sample?The two (theoretical) distributions from which samples are drawn are assumed to have the same SD. This is the value to plug in. -- O__ ---- Peter Dalgaard Blegdamsvej 3 c/ /''_ --- Dept. of Biostatistics 2200 Cph. N (*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918 ~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk) FAX: (+45) 35327907