[R] Sampling in R
Hi all! I was wondering if you can help me with a little sampling issue I'm having in R. I have a database with 100 observations. I need to sample n=9 sample size, 200 times (i.e. get 200 samples of size 9). N (pop. size) =100 Each sample can't contain the same observation more than one time (i.e. the program needs to check if the obs. has already been sampled into the sample - it it has, it should put it back and sample again another obs.) obviously I need to do this with replacement. Then, I need to draw (I think the best is with a histogram) the distribution of the mean of each os the 200 samples. I guess I need to do a loop for the 'sample' function in R. I could really use your help. Regards, Noam Friedman. __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
Re: [R] Sampling in R
Does the following what you want:
x - 1:100
s - matrix(0, nrow=200, ncol=9)
for (i in 1:200) {
s[i, ] - sample(x, 9)
}
m - rowMeans(s)
hist(m)
The default behavior of sample is without replacement.
Thomas
Noam Friedman wrote:
Hi all!
I was wondering if you can help me with a little sampling issue I'm
having in R.
I have a database with 100 observations.
I need to sample n=9 sample size, 200 times (i.e. get 200 samples of
size 9).
N (pop. size) =100
Each sample can't contain the same observation more than one time
(i.e. the program needs to check if the obs. has already been sampled
into the sample - it it has, it should put it back and sample again
another obs.)
obviously I need to do this with replacement.
Then, I need to draw (I think the best is with a histogram) the
distribution of the mean of each os the 200 samples.
I guess I need to do a loop for the 'sample' function in R.
I could really use your help.
Regards,
Noam Friedman.
__
R-help@stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide!
http://www.R-project.org/posting-guide.html
__
R-help@stat.math.ethz.ch mailing list
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PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
Re: [R] Sampling in R
On 3/23/06 5:52 AM, Noam Friedman [EMAIL PROTECTED] wrote:
Hi all!
I was wondering if you can help me with a little sampling issue I'm
having in R.
I have a database with 100 observations.
I need to sample n=9 sample size, 200 times (i.e. get 200 samples of
size 9).
N (pop. size) =100
Each sample can't contain the same observation more than one time
(i.e. the program needs to check if the obs. has already been sampled
into the sample - it it has, it should put it back and sample again
another obs.)
obviously I need to do this with replacement.
Then, I need to draw (I think the best is with a histogram) the
distribution of the mean of each os the 200 samples.
I guess I need to do a loop for the 'sample' function in R.
That sounds correct.
I could really use your help.
x - rnorm(100)
meanvec - vector()
for (j in 1:200) {
y - sample(x,9)
meanvec[j] - mean(y)
}
hist(meanvec)
If this example code doesn't get you started, then you will probably need to
be more specific about where you are getting stuck.
Sean
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Re: [R] Sampling in R
I prefer the function resample as it is given in the sample help, in this
way:
resample - function(x, size, ...)
{
if length(x = 1) {if (!missing(size) size == 0) x[FALSE] else x}
else sample(x, size, ...)
}
Anyway, if an observation can not be twice in the sample, you have to use
replace=FALSE, this is, not replacement.
The solution of Thomas is very good, although you have to check if your data
is going to be in the matrix as rows vectors or columns vector. If the
former, user byrow=TRUE in the matrix function, if the later, thomas
solution is perfect.
Instead of using a for loop, I prefer:
t(replicate(100, resample(x, 9))) if you want a matrix where the vectors are
rows
or replicate(100, resample(x, 9)) if you want a matrix where the vectors are
columns.
So the program can be:
hist(rowMeans(replicate(100, resample(x, 9.
I hope this is useful for you!!
Thanks Regards,
Pedro Canadilla
Oracle DBA
On 3/23/06, Thomas Petzoldt [EMAIL PROTECTED] wrote:
Does the following what you want:
x - 1:100
s - matrix(0, nrow=200, ncol=9)
for (i in 1:200) {
s[i, ] - sample(x, 9)
}
m - rowMeans(s)
hist(m)
The default behavior of sample is without replacement.
Thomas
Noam Friedman wrote:
Hi all!
I was wondering if you can help me with a little sampling issue I'm
having in R.
I have a database with 100 observations.
I need to sample n=9 sample size, 200 times (i.e. get 200 samples of
size 9).
N (pop. size) =100
Each sample can't contain the same observation more than one time
(i.e. the program needs to check if the obs. has already been sampled
into the sample - it it has, it should put it back and sample again
another obs.)
obviously I need to do this with replacement.
Then, I need to draw (I think the best is with a histogram) the
distribution of the mean of each os the 200 samples.
I guess I need to do a loop for the 'sample' function in R.
I could really use your help.
Regards,
Noam Friedman.
__
R-help@stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide!
http://www.R-project.org/posting-guide.html
__
R-help@stat.math.ethz.ch mailing list
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PLEASE do read the posting guide!
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Re: [R] Sampling in R
From: Sean Davis
On 3/23/06 5:52 AM, Noam Friedman [EMAIL PROTECTED] wrote:
Hi all!
I was wondering if you can help me with a little sampling issue I'm
having in R.
I have a database with 100 observations.
I need to sample n=9 sample size, 200 times (i.e. get 200
samples of
size 9). N (pop. size) =100
Each sample can't contain the same observation more than one time
(i.e. the program needs to check if the obs. has already
been sampled
into the sample - it it has, it should put it back and sample again
another obs.)
obviously I need to do this with replacement.
Then, I need to draw (I think the best is with a histogram) the
distribution of the mean of each os the 200 samples.
I guess I need to do a loop for the 'sample' function in R.
That sounds correct.
I could really use your help.
x - rnorm(100)
meanvec - vector()
for (j in 1:200) {
y - sample(x,9)
meanvec[j] - mean(y)
}
hist(meanvec)
If the samples are not needed beyond the calculations Noam mentioned,
then this might save some memory:
samp.means - replicate(200, mean(sample(x, 9)))
hist(samp.means)
Andy
If this example code doesn't get you started, then you will
probably need to be more specific about where you are getting stuck.
Sean
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http://www.R-project.org/posting-guide.html
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[R] Sampling With R
Hello, I'm going to use R to analyze some point count data and I need to use the sample function. Let's say that I have a data set like below with each row being a visit to a site and the numbers in the rows individual distances. The real data set will of course have many more rows. a [[1]] [1] 1 10 2 32 43 54 65 NA [[2]] [1] 2 1 32 6 5 44 3 2 [[3]] [1] 3 3 22 56 7 NA NA NA [[4]] [1] 4 23 4 33 NA NA NA NA [[5]] [1] 5 22 12 2 2 2 32 NA [[6]] [1] 6 22 32 43 23 5 NA NA I'm going to analyze how density changes with number of visits. So I will sample 1 visit (row) x number of times and export the distances to the program DISTANCE. I know that I can do the sampling part using: sample(a,x,replace=TRUE,prob=NULL) This works fine. But I also need to sample 2 visits (rows) out of visits 1-3 and then 2 visits out of visits 4-6, and so on x number of times. Any ideas? Thanks much, Nathan Cooper Utah Division of Wildlife Resources [EMAIL PROTECTED] _ Use custom emotions -- try MSN Messenger 6.0! http://www.msnmessenger-download.com/tracking/reach_emoticon __ [EMAIL PROTECTED] mailing list https://www.stat.math.ethz.ch/mailman/listinfo/r-help
