Any methods or advices about the random sampling method?
I have no idea.
Thanks a lot.
On 7/8/07, Zhang Jian <[EMAIL PROTECTED]> wrote:
>
> The method can get one new data. But I think that it is not random. I used
> the new "random" data to compute the index which I want to get. The
> same value was achieved with the data "sites".
> I try it again and again. The result is the same.
> So I think I need to find one new random sampling method.
>
>
>
> On 7/7/07, Daniel Nordlund <[EMAIL PROTECTED]> wrote:
> >
> > > -----Original Message-----
> > > From: [EMAIL PROTECTED] [mailto:
> > [EMAIL PROTECTED]
> > > On Behalf Of Zhang Jian
> > > Sent: Saturday, July 07, 2007 12:31 PM
> > > To: r-help
> > > Subject: [R] random sampling with some limitive conditions?
> > >
> > > I want to gain thousands of random sampling data by randomizing the
> > > presence-absence data. Meantime, one important limition is that the
> > row and
> > > column sums must be fixed. For example, the data "tst" is following:
> > > site1 site2 site3 site4 site5 site6 site7 site8 1 0 0 0 1 1 0 0 0 1
> > 1 1 0
> > > 1 0 1 1 0 0 0 1 0 1 0 0 0 0 1 0 1 0 1 1 0 1 0 0 0 0 0 0 1 0 1 1 1 1 1
> > 1 0 0
> > > 0 0 0 0 0 0 0 0 1 0 1 0 1
> > >
> > > sum(tst[1,]) = 3, sum(tst[,1])=4, and so on. When I randomize the
> > data, the
> > > first row sums must equal to 3, and the first column sums must equal
> > to 4.
> > > The rules need to be applied to each row and column.
> > > How to get the new random sampling data? I have no idea.
> > > Thanks.
> > >
> >
> > You could reorder your table by stepping through your table a column at
> > a time, and for each column randomly deciding to swap the current column
> > with a column that has the same column total. Repeat this process for each
> > row, i.e. for each row, randomly choose a row with the same row total to
> > swap with.
> >
> > Here is some example code which is neither efficient nor general, but
> > does demonstrate the basic idea. You will need to decide if this approach
> > meets you needs.
> >
> > # I created a data file with your table (8x8) and read from it
> > sites <- read.table("c:/R/R-examples/site_random_sample.txt",
> > header=TRUE)
> > sites
> > # get row and column totals
> > colsums <- apply(sites,2,sum)
> > rowsums <- apply(sites,1,sum)
> > # randomly swap columns
> > for(i in 1:8) {
> > if (runif(1) > .5) {
> > swapcol<-sample(which(colsums==colsums[i]),1)
> > temp<-sites[,swapcol]
> > sites[,swapcol]<-sites[,i]
> > sites[,i]<-temp
> > }
> > }
> > # randomly swap rows
> > for(i in 1:8) {
> > if (runif(1) > .5) {
> > swaprow<-sample(which(rowsums==rowsums[i]),1)
> > temp<-sites[swaprow,]
> > sites[swaprow,]<-sites[i,]
> > sites[i,]<-temp
> > }
> > }
> > sites
> >
> >
> > Hope this is helpful,
> >
> > Dan
> >
> > Daniel Nordlund
> > Bothell, WA USA
> >
> >
> >
> >
>
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