Try adding replace=TRUE to your call to sample, then you will get numbers closer to what you are expecting.
-- Gregory (Greg) L. Snow Ph.D. Statistical Data Center Intermountain Healthcare greg.s...@imail.org 801.408.8111 > -----Original Message----- > From: r-help-boun...@r-project.org [mailto:r-help-boun...@r- > project.org] On Behalf Of Jim Bouldin > Sent: Thursday, July 23, 2009 12:00 PM > To: r-help@r-project.org > Subject: [R] Random # generator accuracy > > > Dan Nordlund wrote: > > "It would be necessary to see the code for your 'brief test' before > anyone > could meaningfully comment on your results. But your results for a > single > test could have been a valid "random" result." > > I've re-created what I did below. The problem appears to be with the > weighting process: the unweighted sample came out much closer to the > actual > than the weighted sample (>1% error) did. Comments? > Jim > > > x > [1] 1 2 3 4 5 6 7 8 9 10 11 12 > > weights > [1] 1 1 1 1 1 1 2 2 2 2 2 2 > > > a = mean(replicate(1000000,(sample(x, 3, prob = weights))));a # (1 > million samples from x, of size 3, weighted by "weights"; the mean > should > be 7.50) > [1] 7.406977 > > 7.406977/7.5 > [1] 0.987597 > > > b = mean(replicate(1000000,(sample(x, 3))));b # (1 million samples > from > x, of size 3, not weighted this time; the mean should be 6.50) > [1] 6.501477 > > 6.501477/6.5 > [1] 1.000227 > > > Jim Bouldin, PhD > Research Ecologist > Department of Plant Sciences, UC Davis > Davis CA, 95616 > 530-554-1740 > > ______________________________________________ > R-help@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting- > guide.html > and provide commented, minimal, self-contained, reproducible code. ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
