On Wed, 27 Feb 2008, juli pausas wrote:
> Dear all,
> I have a question on glm, family binomial. I do not see significant
> differences between the levels of a factor (treatment) if all data for
> a level is 0; and replacing a 0 for a 1 (in fact reducing the
> difference), then I detect the significant difference that I expected.
This is because you are using the wrong test, one with negligible power.
See MASS4 pp.197-8 -- you need to use the LRT, as in
> drop1(glm(sf ~ tr, family="binomial"), test="Chisq")
Single term deletions
Model:
sf ~ tr
Df Deviance AIC LRT Pr(Chi)
<none> 1.595 17.730
tr 1 24.244 38.379 22.649 1.944e-06
(and in your example you can replace 'drop1' by 'anova').
> Is there a way to overcome this problem? or this is an expected
> behaviour ? Here is an example:
>
> s <- c(2,4,4,5,0,0,0,0)
> f <- c(31,28,28,28,32,37,34,35)
> tr <- gl(2, 4)
> sf <- cbind(s,f) # numbers of successes and failures
> summary(glm(sf ~ tr, family="binomial")) # tr ns
>
> sf[8,1] <- 1
> summary(glm(sf ~ tr, family="binomial")) # tr significative **
>
> Thanks for any suggestion
>
> Juli
>
> --
> http://www.ceam.es/pausas
>
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--
Brian D. Ripley, ripley at stats.ox.ac.uk
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
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