R help - Mar 2006 - fitting a distribution using glm

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.
cheers, vumani

"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             ?ster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
 (*) \(*) -- University of Copenhagen   Denmark          Ph:  (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)                  FAX: (+45) 35327907

On Wed, 8 Mar 2006, Vumani Dlamini wrote:
> 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.
Yes. Easy.

  glm(poisdata ~ 1, family = poisson)
>
> 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, they would be.  I don't know what this is doing, but it certainly 
isn't fitting a poisson distribution to poisdata.

 	-thomas