Dear friends,
 suppose my dataset is the following data:

id<-1:9
x<-c(1,2,3,1,2,3,1,2,3)
y<-c(1,1,1,2,2,2,3,3,3)
data<-data.frame(id,x,y)

     id  x   y
1  1   1   1
2  2   2   1
3  3   3   1
4  4   1   2
5  5   2   2
6  6   3   2
7  7   1   3
8  8   2   3
9  9   3   3
i want to do sampling like this:say the sample size is 3.
 First: random sampling from x;
Next ,random sampling from y ;and combing sampled x and sampled y;
Finally, output the samples: id x and y.
I think i could call it two-dimension sampling.
Thanks very much!



-- 
Kind Regards,
Zhi Jie,Zhang ,PHD
Department of Epidemiology
School of Public Health
Fudan University
Tel:86-21-54237149

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