Hi,
Thanks for responding/noticing. I was going to wait a few more days
while contemplating how to get more notice for the post.
It occurred to me later that it should have gone to R-devel as you
have forwarded it. I'll include my example (with a fix) for refresher
c1 <- structure(list(Contr = c(0.028, 0.043, 0.064, 0.097, 0.146, 0.219
), Correct = c(34L, 57L, 94L, 152L, 160L, 160L), Incorrect = c(126L,
103L, 66L, 8L, 0L, 0L)), .Names = c("Contr", "Correct", "Incorrect"
), row.names = c(NA, 6L), class = "data.frame")
glm(cbind(Correct, Incorrect) ~ Contr - 1, binomial,
data = c1, offset = qlogis(0.25))
Error in model.frame.default(formula = cbind(Correct, Incorrect) ~ Contr - :
variable lengths differ (found for '(offset)')
lm(cbind(Correct, Incorrect) ~ Contr - 1, binomial,
data = c1, offset = rep(qlogis(0.25), nrow(c1))) ## which works
The line which throws the error is:
mf <- eval(mf, parent.frame())
I must have done something additional of which I did not keep a record,
but if I change the nrow(Y) in the code fragment to nrow(data),
that works, as in
if (!is.null(offset)) {
if (length(offset) == 1)
offset <- rep(offset, NROW(data))
else if (length(offset) != NROW(data))
stop(gettextf("number of offsets is %d should equal %d
(number of observations)",
length(offset), NROW(data)), domain = NA)
}
mf$offset <- offset
mf <- eval(mf, parent.frame())
Thank you.
Ken
Quoting Heather Turner <Heather.Turner at warwick.ac.uk>:
This post about length 1 offsets on R help seems to have been ignored
(sorry deleted original email - is there a way to continue thread in
this case?)
https://stat.ethz.ch/pipermail/r-help/2009-February/189352.html
It does seem to be a bug, in that glm does not behave as documented. In
fact the same bug applies to lm as well.
I don't think the suggested fix works though - Y isn't available until
the model frame is evaluated. Moreover you would want to evaluate the
offset as part of the model frame, to take care of the search path,
subset and na.action.
One possibility would be to modify model.frame to recycle variables of
length one. This would be a potentially useful feature from my point of
view as offset(constant) could replace Const(constant) in gnm, which is
basically a work around for this problem in the case of specifying a
constant in a nonlinear term. However, this solution may have undesired
side-effects and I don't feel too strongly about it as there are various
work-arounds. Perhaps the simplest solution would be to modify the docs
so that offsets of length one are disallowed - it is easy enough for the
user to create a vector of the right length as necessary.
Best regards,
Heather
Ken Knoblauch
Inserm U846
Stem-cell and Brain Research Institute
Department of Integrative Neurosciences
18 avenue du Doyen L?pine
69500 Bron
France
tel: +33 (0)4 72 91 34 77
fax: +33 (0)4 72 91 34 61
portable: +33 (0)6 84 10 64 10
http://www.sbri.fr/members/kenneth-knoblauch.html