Underdispersion with anova testing methods
Using anova of a glm with test = "Chisq", I get this:
Analysis of Deviance Table
Model: poisson, link: log
Response: Days
Terms added sequentially (first to last)
Df Deviance Resid. Df Resid. Dev P(>|Chi|)
NULL 373 370.56
Block 3 71.05 370 299.51 2.543e-15
Variety 1 94.04 369 205.47 3.096e-22
Instar 3 126.21 366 79.26 3.553e-27
Variety:Instar 3 7.74 363 71.53 0.05
Other interactions were much less prominent. I'm only interested in
the effect of Variety, and one would expect an effect of Instar, so I
want to be able to block on Instar and Block.
Evidently, the Chisquare test does not use an estimate of the
dispersion parameter. Were it included, the interaction would have a
much smaller probability. Is there a good reason why one should or
should not makes such an adjustment? In any case, will it matter when
I'm interested only in the effect of Variety?
Alternatively, could the underdispersion come from my ignoring the
fact that the insects are measured at the four different instars and
so the independence assumption is not true. I could not think of a way
of taking that lack of independence into account.
Suggestions welcome.
Thanks
*************************************************************
___ Patrick Connolly
{~._.~} HortResearch Great minds discuss ideas;
_( Y )_ Mt Albert Average minds discuss events;
(:_~*~_:) Auckland Small minds discuss people.
(_)-(_) New Zealand .... Anon
Ph: +64-9 815 4200 x 7188
*************************************************************
______________________________________________________
The contents of this e-mail are privileged and/or confidential to the
named recipient and are not to be used by any other person and/or
organisation. If you have received this e-mail in error, please notify
the sender and delete all material pertaining to this e-mail.
______________________________________________________
-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html
Send "info", "help", or "[un]subscribe"
(in the "body", not the subject !) To: r-help-request at stat.math.ethz.ch
_._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._