In discussing permutation tests, _Modern Applied Statistics With S Plus_ suggests this (IMHO) rather elegant approach to generating a distribution of t-tests: d<- ...some data... ttest <- function(x) mean(x)/sqrt(var(x)/length(x)) n<-1000 res<-numeric(n) for(i in 1:n) res[i] <- ttest(x<-d*sign(runif(10)-0.5) The problem is that the sign(runif) results in random permutation of the observations into two groups of random size. For my analysis, however, I want to fix the size of the two groups. So I wonder if someone can point me toward an elegant way to randomly split a vector into two groups of specified sizes. Or, similarly, randomly select a specific number of elements from a vector. Thanks, Eric -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.- r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html Send "info", "help", or "[un]subscribe" (in the "body", not the subject !) To: r-help-request at stat.math.ethz.ch _._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._
Select exactly n elements of a vector
2 messages · Kort, Eric, Brian Ripley
On Mon, 12 Aug 2002, Kort, Eric wrote:
In discussing permutation tests, _Modern Applied Statistics With S Plus_ suggests this (IMHO) rather elegant approach to generating a distribution of t-tests: d<- ...some data... ttest <- function(x) mean(x)/sqrt(var(x)/length(x)) n<-1000 res<-numeric(n) for(i in 1:n) res[i] <- ttest(x<-d*sign(runif(10)-0.5) The problem is that the sign(runif) results in random permutation
of the observations into two groups of random size. For my analysis, however, I want to fix the size of the two groups. So I wonder if someone can point me toward an elegant way to randomly split a vector into two groups of specified sizes. Or, similarly, randomly select a specific number of elements from a vector. That's part of sample(). sample(10,4, TRUE) selects 4 elements from 1:10. (and the negation will extract the other sample).
Brian D. Ripley, ripley at stats.ox.ac.uk Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272860 (secr) Oxford OX1 3TG, UK Fax: +44 1865 272595 -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.- r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html Send "info", "help", or "[un]subscribe" (in the "body", not the subject !) To: r-help-request at stat.math.ethz.ch _._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._