On 23/07/2009 2:48 PM, Jim Bouldin wrote:
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.
I think you are looking in the wrong place. Most often (as for sample!)
they just list the header of the function:
sample(x, size, replace = FALSE, prob = NULL)
and the default is explicit: "replace = FALSE". Sometimes this is
repeated in the text, and sometimes it is only in the text, but there
are very few cases where a default is defined but not documented, and I
think those qualify as documentation errors that should be fixed.
Duncan Murdoch
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
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R-help@r-project.org mailing list
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PLEASE do read the posting guide http://www.R-project.org/posting-
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Jim Bouldin, PhD
Research Ecologist
Department of Plant Sciences, UC Davis
Davis CA, 95616
530-554-1740
______________________________________________
R-help@r-project.org mailing list
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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
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
