[R] glm binomial with zero proportions
Gregor Gorjanc
gregor.gorjanc at bfro.uni-lj.si
Fri Mar 3 09:32:22 CET 2006
Hello!
>> Hello. I must fit a logistic regression to data in the form of
>> proportions, but in which some of the proportions are zero. I therefore
>> cannot use the glm function with a binomial link since the link function
>> is not defined for p=0 or 1.
>
> It is a logistic link, not a 'binomial link'. The fitted proportions
> cannot be zero (just 2*10^-16 which is exceedingly rare), but the observed
> ones can. MASS4 p.190 has an example with observed proportion zero.
>
Yes, MASS4 p. 190 has example with 0 observed, but that is not a problem
if you use a regression for dose. What can one do in case you have a
level of a factor that has observed zero proportion or very low/high
proportion?
>> What other solutions are available? Any
>> references to this specific problem (i.e. regression using proportions,
>> of which some are zero) would be welcome.
>
> Any good book on logistic regression.
--
Lep pozdrav / With regards,
Gregor Gorjanc
----------------------------------------------------------------------
University of Ljubljana PhD student
Biotechnical Faculty
Zootechnical Department URI: http://www.bfro.uni-lj.si/MR/ggorjan
Groblje 3 mail: gregor.gorjanc <at> bfro.uni-lj.si
SI-1230 Domzale tel: +386 (0)1 72 17 861
Slovenia, Europe fax: +386 (0)1 72 17 888
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"One must learn by doing the thing; for though you think you know it,
you have no certainty until you try." Sophocles ~ 450 B.C.
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