[R] Random # generator accuracy

[Jim Bouldin]

Thanks Greg, that most definitely was it.  So apparently the default is
sampling without replacement.  Fine, but this brings up a question I've had
for a bit now, which is, how do you know what the default settings are for
the arguments of any given function?  The HTML help files don't seem to
indicate in many (most) cases.  Thanks. 

> 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.snow at imail.org
> 801.408.8111
> 
> 
> > -----Original Message-----
> > From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> > project.org] On Behalf Of Jim Bouldin
> > Sent: Thursday, July 23, 2009 12:00 PM
> > To: r-help at 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
> > 
> > ______________________________________________
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> > PLEASE do read the posting guide http://www.R-project.org/posting-
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> > and provide commented, minimal, self-contained, reproducible code.
> 

Jim Bouldin, PhD
Research Ecologist
Department of Plant Sciences, UC Davis
Davis CA, 95616
530-554-1740