[R] fitting a distribution using glm

Peter Dalgaard p.dalgaard at biostat.ku.dk
Wed Mar 8 15:32:11 CET 2006


"Vumani Dlamini" <dvumani at hotmail.com> writes:

> it is easy to fit a distribution using fitdistr
> 
> poisdata <- rpois(n = 100, lambda = 2)
> poismle <- fitdistr(poisdata, "Poisson")
> poismle
> 
> but i would like to know whether its possible to get an identical result 
> using glm. I use
> 
> poistab <- data.frame(table(poisdata))
> colnames(poistab) <- c("width","freq");
> poistab[,"width"] <- as.numeric(poistab[,"width"])
> glm(freq ~ 1-width, family = poisson, data = poistab)
> 
> but the results are always different.


Well, the freq values in that setup are not independent Poisson
variates. For a start, they sum to constant!

Try instead

exp(coef(glm(poisdata~1,family=poisson)))


-- 
   O__  ---- Peter Dalgaard             Øster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
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~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)                  FAX: (+45) 35327907




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