R help - Jul 2011 - GLM different results with the same factors

I've read something about this problem, but I don't know how can i avoid
this
problem.
Why the order of the factors give different results? I suppose it's because
the order of the factors, i've just changed "lcc" from the first
position to
the last in the model, and the significance change completely 
> modo<-glm(prevalencia~*lcc*+edadysexo/lcc+edadysexo/mes,binomial)
> anova(modo,test="Chisq")
              Df Deviance Resid. Df Resid. Dev P(>|Chi|)   
NULL                            524     206.97             
lcc            1  10.5715       523     196.40  0.001148 **
edadysexo      2   1.0725       521     195.32  0.584929   
lcc:edadysexo  2   3.7752       519     191.55  0.151433   
edadysexo:mes  9  18.2981       510     173.25  0.031868 * 
> mode<-glm(prevalencia~edadysexo/lcc+edadysexo/mes+*lcc*,binomial)
> anova(mode,test="Chisq")
              Df Deviance Resid. Df Resid. Dev P(>|Chi|)   
NULL                            524     206.97             
edadysexo      2   9.9165       522     197.05  0.007025 **
lcc            1   1.7275       521     195.32  0.188732   
edadysexo:lcc  2   3.7752       519     191.55  0.151433   
edadysexo:mes  9  18.2981       510     173.25  0.031868 * 

Ijow can i know what's correct? when i test this two factors separately in a
lm (lcc is continuos) and in a chisq.test (edadysexo is categorical) both
are significant, and in the model just one of them is significant.

Thanks very much


-----
Mario Garrido Escudero
PhD student
Dpto. de Biolog?a Animal, Ecolog?a, Parasitolog?a, Edafolog?a y Qca. Agr?cola
Universidad de Salamanca
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On Jul 24, 2011 Gaiarrido wrote:
> Why the order of the factors give different results? I suppose it's
> because the order of the 
> factors, i've just changed "lcc" from the first position to
the last in
> the model, and the significance 
> change completely
> ...snip <
> Ijow can i know what's correct? 
Both are correct. R's default anova uses sequential sums of squares and so
tests model terms sequentially. Reordering the factors in your model
therefore leads to different hypotheses being tested. You are looking for
what are often called Type II tests. To get them use drop1() on your glm
object or install the car package and use its Anova function.

Regards, Mark.

-----
Mark Difford (Ph.D.)
Research Associate
Botany Department
Nelson Mandela Metropolitan University
Port Elizabeth, South Africa
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