[R] type III effect from glm()

[(Ted Harding)]

On 19-Feb-09 10:38:50, Simon Pickett wrote:
> Hi all,
> 
> This could be naivety/stupidity on my part rather than a problem
> with model output, but here goes....
> 
> I have fitted a fairly simple model 
> 
> m1<-glm(count~siteall+yrs+yrs:district,family=quasipoisson,
>         weights=weight,data=m[x[[i]],])
> 
> I want to know if yrs (a continuous variable) has a significant
> unique effect in the model, so I fit a simplified model with the
> main effect ommitted...
> 
> m2<-glm(count~siteall+yrs:district,family=quasipoisson,
>         weights=weight,data=m[x[[i]],])

So, above, you have fitted two models: m1, m2

> then compare models using anova()
> anova(m1,m1b,test="F")

And here you are comparing two models: m1, m1b

Could this be the reason for your result?

> Analysis of Deviance Table
> 
> Model 1: count ~ siteall + yrs + yrs:district
> Model 2: count ~ siteall + yrs:district
>   Resid. Df Resid. Dev   Df Deviance F Pr(>F)
> 1      1936      75913                       
> 2      1936      75913    0        0         
> 
> The d.f.'s are exactly the same, is this right? Can I only test the
> significance of a main effect when it is not in an interaction? 
> 
> Thanks in advance,
> Simon.

--------------------------------------------------------------------
E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
Fax-to-email: +44 (0)870 094 0861
Date: 19-Feb-09                                       Time: 10:56:12
------------------------------ XFMail ------------------------------