[R] overdispersion and quasibinomial model

[Ben Bolker]

djpren wrote:
> 
> Thanks for the reply. Naturally I already searched the site and help for
> the answers to these questions. I think I've figured out how to run a
> quasi-binomial model, but I cannot figure out how to test for
> over-dispersion or how to apply a shapiro-wilk test.
> 
> This is not homework, neither do I have an instructor who is proficient in
> using R. This program was suggested to me by another researcher after he
> witnessed my frustration with the inflexibility of SPSS and other such
> programs. I am on a very tight schedule and I don't have time to become a
> statistician and computer scientist, which is why I wrote 3 very quick
> questions asking for commands that i had already tried to find myself.
> 
> Testing for over-dispersion is probably something I can eventually get to
> grips with, since I just have get variance for the real and modelled data.
> However, I cannot find a command to do shapiro-wilks on the site or on
> these forums. Also, why do you say that most people here wouldn't
> recommend this procedure?
> 
> 

??shapiro
stats::shapiro.test     Shapiro-Wilk Normality Test

(maybe you were searching for "shapiro-wilks" (sic)?)

People often disrecommend statistical tests of normality because they 
have low power for small data sets (hence you don't have power to
detect non-normality when it is present) and high power for large
data sets even when the degree of non-normality detected is not
enough to invalidate the results of some statistical procedures.
Under what circumstances are the residuals from a quasibinomial
GLM expected to be normally distributed ... ?







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