Selecting a random sample

Mon, May 22, 2000 11:57 AM #

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:

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?

Thanks in advance,

Scot


--
  Scot W. McNary  email:smcnary at charm.net   

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Brian Ripley

Mon, May 22, 2000 12:37 PM #

On Mon, 22 May 2000, Scot W McNary wrote:

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

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