Selecting a random sample
On Mon, 22 May 2000, Scot W McNary wrote:
Hi, I'm trying to learn R and get an errand done at the same time. I have a list of cases with ids numbered sequentially 1 to 403. I want to select at random exactly 20 without replacement. Here's what I've tried so far:
ids<-(1:403) pick<-runif(403) picklist<-as.data.frame(cbind(ids,pick)) who<-subset(picklist, pick<20/403) who
This works, but I can have anywhere from 16 to 32 cases in the subset 'who', due to random variability in the runif generator. What's a more efficient (and general) way to extract a subset of exactly n from a total sample N?
sample(N, n, replace=F) Easy? See ?sample for variations on the theme. By the way, seq(N)[runif(N) < n/N] would have done it your way in simpler code, and seq(N)[rank(runif(N)) <= n] is another general solution.
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 _._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._