Dead R developers,
In R-devel 2023-02-11 and older R versions, there is a note in the "lm
{stats}" help page specifying that:
> Offsets specified by offset will not be included in predictions by
> predict.lm<http://127.0.0.1:12534/library/stats/help/predict.lm>, whereas
> those specified by an offset term in the formula will be.
However, the source code as well as basic tests seem to show that both types of
offset terms are always used in predictions.
a<-data.frame(off=1:4, outcome=4:1)
mod<-lm(data=a, outcome~1, offset=off)
coef(a) # intercept is zero
predict(mod) # returns 1:4, which uses offset
predict(mod, newdata=data.frame(off=c(3,2,5))) # returns c(3,2,5) which uses
the new offset
When looking at the history of R source code, this note seems to exist from R
1.0.0 while the source code of predict.lm already called
eval(object$call$offset, newdata)
https://github.com/SurajGupta/r-source/blob/1.0.0/src/library/base/R/lm.R
https://github.com/SurajGupta/r-source/blob/1.0.0/src/library/base/man/lm.Rd
Version 0.99.0 did not contain the note, but already had the call to
eval(object$call$offset, newdata)
https://github.com/SurajGupta/r-source/blob/0.99.0/src/library/base/man/lm.Rd
https://github.com/SurajGupta/r-source/blob/0.99.0/src/library/base/R/lm.R
The actual behavior of R seems to be sane to me, but unless I miss something,
this looks like a documentation bug.
It seems to have bugged someone before:
https://stackoverflow.com/questions/71264495/why-is-predict-not-ignoring-my-offset-from-a-poisson-model-in-r-no-matter-how-i
Digging deeper in R history, it seems that this note was also found in "glm
{stats}" in R 1.0.0 but was removed in R 1.4.1. Maybe somebody forgot to remove
it in "lm {stats}" too.
--
Sincerely
Andr� GILLIBERT
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