[R] Question about computing offsets automatically
Louisell, Paul T.
paul.louisell at pw.utc.com
Thu Nov 6 20:33:26 CET 2003
Hi,
I'm using R version 1.8.0 on Windows NT. When fitting a glm with Poisson
random component and a log link, I frequently need to include an offset.
Typically I use xtabs or table to get the counts for the contingency table,
and then I use as.data.frame.table to create a data frame that I can use in
the glm function. I have not found an option that allows me to total the
offset variable to obtain offsets for cells in the contingency table.
For example, suppose I have the following data frame named Data:
F1 F2 Off
1 A C 4
2 A C 3
3 A C 2
4 B C 3
5 A D 2
6 A D 4
7 B D 1
xtabs(~F1+F2, data=Data) produces the contingency table:
F2
F1 C D
A 3 2
B 1 1
And as.data.frame.table(xtabs(~F1+F2, data=Data)) changes the contingency
table to a data frame suitable for use in the glm function:
F1 F2 Freq
1 A C 3
2 B C 1
3 A D 2
4 B D 1
What I'm looking for is some option that would add a 4th column to the
output of as.data.frame.table which contains the offsets for each cell in
the contingency table:
F1 F2 Freq Off
1 A C 3 9
2 B C 1 3
3 A D 2 6
4 B D 1 1
Does such an option exist somewhere in R (I wasn't able to find it in the
documentation for the table, xtabs, as.data.frame.table, or glm functions)?
I can obtain the Off column easily enough in a simple loop, but I thought
there might be an option for this somewhere.
Paul Louisell
Statistician
(860) 565-5417
louisept at pweh.com
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