[S] by() function

Weiguo Fan (fanwg@iscs.nus.edu.sg)
Wed, 6 May 1998 18:29:30 +0800 (GMT-8)

Dear S-PLUS users,

I once posted a message regarding applying regression function to
crosstabulated data set. For example, in data frame DATA, one factor A1
has 4 levels, another A4 has 6 levels, so in combination, there should be
24 levels.

Several people has suggested me to use the by() function.

I used by() to get the first result.

Then I manually split the DATA frame to 24 subsets and applyed the
function to each of them. I got the second result.

To my surprise, these two results are quite different.

Can anybody tell me how is by() implemented in S-PLUS? How to explain
such a difference? Which approach should be adopted in real practice?

Thanks for any suggestion!



Mr. Weiguo Fan
Department of Information Systems and Computer Science
National University of Singapore
Lower Kent Ridge Road
Singapore 119260

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