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In "A", rows 1-1000, I generate random numbers using RAND(). I want to pick numbers between say 1-100.

In "B" for 1000 rows I have =RAND()*(100-1)+1

In "C" for 1000 rows I use the numbers that I generated in "A", so =A1*(100-1)+1

The averages in B and C are the same, as expected.

I only want to use the values for 20% of the time, so for each of the 1000 rows:

For B, I use =IF(A1<0.2,B1,0)
For C, I use =IF(A1<0.2,C1,0)

If I do this, the averages for each of the two columns are vastly different.

Can anybody explain why?

Both approaches are using random numbers. The first approach uses two different random numbers (from column "A" and the one used in the "B" column calculation).

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Columns A and B have the same uniform distribution, aside from the linear transformation, but are independant. B's probability density function (PDF) for A<0,2 is the same as the PDF for any other selection of A.

Columns A and C are not independant. In fact, they are completely deterministic. If you select rows where A<0,2, you are only selecting rows where C<20,8. The probability of finding a value of C greater than 20,8 within that selection has dropped to zero. Clearly, the distribution has changed.

For a more obvious example of this, consider rolling two dice (A and B) and their total (C). I'd expect to see an average of 3,5 in columns A and B, and an average of 7 in column C. If from this table I only select rows where die A landed on 1, I'd still see an average of 3,5 in B (independant), but merely 4,5 instead of 7 in column C (dependant).

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Thank you very much. This was driving me crazy. Your explanation is very clear. – NMS Mar 28 '13 at 15:29

For your 1 in 5 sample from ColumnC, instead of choosing the C value based on the A value for the corresponding row (ie those A values less than 0.2) try choosing your 1 in 5 C sample from an adjacent row. Say use =IF(A1<0.2,C2,0). This should give you similar averages for the two columns (though =A1(100-1)+1 is not correct).

Then you should have a more-or-less representative 1 in 5 of ColumnC rather than merely (approximately) a scaled up version of values preselected to be the bottom 20%.

In other words, your ColumnB ‘vastly different’ (but probably anticipated ie around 50) average is probably about five times the ColumnC sample average. Choose 1 in 10 on the above basis and would be ~10 times.

Rather than scaling up, if you want to generate 1000 approximately random numbers in the range 1-100 then you could apply =RANDBETWEEN(1,100).

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