[R] ANOVA-like tests of geometrically-distributed data

[Bill.Venables@cmis.csiro.au]

Robert Merkel asks:

>  -----Original Message-----
> From: 	Robert Merkel [mailto:rmerkel at venus.it.swin.edu.au] 
> Sent:	Tuesday, July 16, 2002 4:05 PM
> To:	r-help at stat.math.ethz.ch
> Subject:	[R] ANOVA-like tests of geometrically-distributed data
> 
> I have a statistical problem which has given me no end of grief recently, 
> and am posting here in the hope that somebody can give me a straight 
> answer.  I'm a IT postgrad, not a statistician, so people may have to 
> speak really slowly and clearly for me to get it :)
	[WNV]  Indeed.  I'm a bit surprised you seem to think that all the
help you need is a bit of quick advice by email.  This stuff can be rather
tricky, like a lot of statistics.

> I am collecting simulation data, and the results are geometrically 
> distributed (or approximately so).  From what I can gather from my stats 
> books, provided the sample size is large enough (>30 or so) I can use t 
> and z-tests to compare means under different experimental conditions as 
> the CLT says that the sample means will be approximately normally 
> distributed.  
> 
> However, also as I understand it, the ANOVA explicitly assumes that the 
> population is normally distributed, which is an assumption that in my case
> 
> is not satisfied.
> 
> I have also been told that something called a "generalized linear model" 
> can be used to perform ANOVA-like statistics on geometrically-distributed 
> data, but not how.  
> 
> There is R documentation on a function "glm" and also "anova.glm" which 
> discuss stuff that looks vaguely like what I want to do, but I can't 
> really make sense of it.
	[WNV]  The geometric distribution is a special case of the negative
binomial, which can indeed be fitted using glm but you need the
negative.binomial( ) function from MASS (or some equivalent) to provide the
family.  You will need to sort out what special value of theta corresponds
to the geometric distribution.  It could be 1, but I'm really not sure.
This information may be useful to whoever can advise you more fully.

	Have fun.

> Can these functions do what I'm trying to do?
	[WNV]  Easy, yes.
>   If so, what's the 
> procedure?
	[WNV]  Much harder to give in detail here.

> Any help will be *much* appreciated.
	[WNV]  Even if the news is not good?

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