[R] Underdispersion with anova testing methods
ripley at stats.ox.ac.uk
Thu Mar 21 09:18:52 CET 2002
On Thu, 21 Mar 2002, Patrick Connolly wrote:
> 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?
1) A poisson family does not, a quasipoisson family does. It's your
choice, but a Poisson distribution does not have a dispersion
parameter so you would be fitting a quasi-model.
2) It's not valid to look at terms in a sequential anova table like this,
as they all interact (and especially at main effects with an interaction
present). Use drop1().
> 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.
Are these small counts? If so, the residual deviance is a badly biased
estimate of the dispersion parameter, so there might not be
under-dispersion at all. See the V&R on-line Statistical Complements
Brian D. Ripley, ripley at stats.ox.ac.uk
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272860 (secr)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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
More information about the R-help