[Rd] ? bug in 'sample' (PR#8813)

[Peter Dalgaard]

doddl at mail.nih.gov writes:

> I have found that specifying different "sizes" in the sample command has
> a funny effect on the random sampling.  The code below is a condensed
> version of a function I wrote to simulate a bootstrap method.  For
> simplicity, I eliminated the internal bootstrap loop, but kept a
> statement to draw one bootstrap sample, because this is where the
> problem occurs.  The output (mean(y)^2) should be the same for the two
> runs of the function below, since it is based on the original rnorm
> draw.  For the first iteration of this function, it is, but thereafter
> the results aren't the same.  If I run this code in Splus, the results
> are the same.  Also, if I set the size argument to be the same (or
> eliminate this line altogether), I get reproducible results.  


What is the issue? To generate samples of different sizes, you may
need a different number of random draws, so the RNG is not going to be
in the same state the 2nd time you hit rnorm().

A current Splus (7.0.0) doesn't give you the same result either:


 > test.fun(50,0,30,12345)
[1] 2.712664e-05 9.971680e-03
> test.fun(50,0,10,12345)
[1] 2.712664e-05 2.225859e-03

>  
> 
> I'm using R 2.2.0
> 
>  
> 
> Thanks,
> 
> -Lori
> 
>  
> 
> test.fun<-function(nsamp, mu,q, seed, nsim=2){
> 
> set.seed(seed)    
> 
> mnsq<-rep(NA,nsim)
> 
> for (i in 1:nsim){
> 
> y<-rnorm(nsamp,mean=mu)
> 
> yboot<-sample(y, size=q, replace=T)
> 
> mnsq[i]<-(mean(y)^2)
> 
> }
> 
> return(mnsq)
> 
> }
> 
>  
> 
> test.fun(50,0,30,12345)
> 
> test.fun(50,0,10,12345)
> 
>  
> 
> Lori E. Dodd, Ph.D.
> 
> National Cancer Institute
> 
> Biometric Research Branch
> 
> 6130 Executive Blvd, MSC 7434
> 
> Rockville, MD 20892 
> 
> (For overnight deliveries use 20852)
> 
>  
> 
> Phone: 301-402-0383
> 
> FAX:    301-402-0560
> 
> Email: doddl at mail.nih.gov
> 
>  
> 
> 
> 	[[alternative HTML version deleted]]
> 
> ______________________________________________
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> 

-- 
   O__  ---- Peter Dalgaard             Øster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
 (*) \(*) -- University of Copenhagen   Denmark          Ph:  (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)                  FAX: (+45) 35327907