Hi,
I have the following data.

> set.seed(6245)
> data <- data.frame(x1=rnorm(5),x2=rnorm(5),x3=rnorm(5),x4=rnorm(5))
> round(data,digits=3)
      x1     x2     x3     x4
1  0.482  1.320 -0.859 -0.142
2 -0.753 -0.041 -0.063  0.886
3  0.028 -0.256 -0.069  0.354
4 -0.086  0.475  0.244  0.781
5  0.690 -0.181  1.274  1.633

What I would like to do is drop 20% of the data. But I want this 20% to
only come from dropping data from x3 and x4. It doesn't have to be evenly,
i.e. I don't care to drop 2 from x3 and 2 from x4 or make sure only one
observation has missing data on only one variable. I just want to drop 20%
of the data through x3 and x4 only.  In other words,

       x1     x2     x3     x4
1  0.482  1.320 -0.859 NA
2 -0.753 -0.041 -0.063  0.886
3  0.028 -0.256      NA  0.354
4 -0.086  0.475      NA  0.781
5  0.690 -0.181      NA  1.633

OR

      x1     x2     x3     x4
1  0.482  1.320     NA -0.142
2 -0.753 -0.041 -0.063  0.886
3  0.028 -0.256      NA  NA
4 -0.086  0.475  0.244  NA
5  0.690 -0.181  1.274  1.633

OR

      x1     x2     x3     x4
1  0.482  1.320 -0.859 -0.142
2 -0.753 -0.041 -0.063     NA
3  0.028 -0.256 -0.069     NA
4 -0.086  0.475  0.244     NA
5  0.690 -0.181  1.274     NA

ETC. are all fine.

Any ideas how I can do this?
Chris

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