[R] Sampling in R

[Liaw, Andy]

From: Sean Davis
> 
> On 3/23/06 5:52 AM, "Noam Friedman" <noamfrie at qos.net.il> 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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