[R] glm binomial with zero proportions

[Gregor Gorjanc]

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

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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
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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.