[R] type III effect from glm()
mark_difford at yahoo.co.uk
Thu Feb 19 13:58:15 CET 2009
>> [On my response] ...not really a sensible question until...
On reading through this...what I mean is that yours seems not to be a
"sensible approach," the question itself may be reasonable. What you want to
be doing is testing whether the interaction term (yrs:district) gets
dropped. Do it by comparing nested models (basically as you have done), or
use dropterm() or stepAIC() [both are in MASS].
Mark Difford wrote:
> Hi Simon,
>>> 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...
> [A different approach...] This is not really a sensible question until you
> have established that there is no significant interaction between "yrs"
> and "district." If this interaction is significant then, ipso facto, the
> effect of "yrs" is not unique but depends on "district." So establish that
> There is a good section on marginality in MASS (Venables & Ripley) and, as
> Mark has mentioned, in Prof Fox's texts. From what I can remember, some of
> these tests are reparametrized behind the scenes to enforce the
> marginality constraint.
> Regards, Mark.
> Simon Pickett-4 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
>> 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
>> then compare models using anova()
>> 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,
>> Dr. Simon Pickett
>> Research Ecologist
>> Land Use Department
>> Terrestrial Unit
>> British Trust for Ornithology
>> The Nunnery
>> [[alternative HTML version deleted]]
>> R-help at r-project.org mailing list
>> PLEASE do read the posting guide
>> and provide commented, minimal, self-contained, reproducible code.
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