[R] GLM different results with the same factors

Mark Difford mark_difford at yahoo.co.uk
Sun Jul 24 18:50:47 CEST 2011

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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