R help - Mar 2010 - test the goodness of it for negative binomial type 2

[code]library(MASS)
x=c(rep(0,8096),
        rep(1,1629),
        rep(2,233),
        rep(3,38),
        rep(4,4)
        )

x.bar=round(mean(x),4)
x.var=round(var(x),4)

p.hat=round(x.bar/x.var,4)
alpha.hat=round(x.bar*p.hat/(1-p.hat),4)

fitdistr(x, "Negative Binomial")
fitdistr(x, "Poisson")[/code]


1- fitdistr(x, "Negative Binomial")
   the parameters got here, is it for negative binomial type 2?
   how can i ask it to use the methods of moments to calculat the
parameters?
   (p.hat and alpha.hat which i derived from methods of moments seem to give
a different value)

2-how can i fit it and than test the goodness of it?

3-then compare with the Poisson model?

Thanks

==============================================
by negative binomial type two i mean 




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On Tue, 2 Mar 2010, casperyc wrote:
>
> [code]library(MASS)
> x=c(rep(0,8096),
>        rep(1,1629),
>        rep(2,233),
>        rep(3,38),
>        rep(4,4)
>        )
>
> x.bar=round(mean(x),4)
> x.var=round(var(x),4)
>
> p.hat=round(x.bar/x.var,4)
> alpha.hat=round(x.bar*p.hat/(1-p.hat),4)
>
> fitdistr(x, "Negative Binomial")
> fitdistr(x, "Poisson")[/code]
>
>
> 1- fitdistr(x, "Negative Binomial")
>   the parameters got here, is it for negative binomial type 2?
Yes.
>   how can i ask it to use the methods of moments to calculat the
> parameters?
You can't with fitdist()
>   (p.hat and alpha.hat which i derived from methods of moments seem to give
> a different value)
It is a different parametrization of NB2 that you are using. Your 
alpha.hat corresponds to "size" and p.hat corresponds to
"prob" in
?dnbinom. However, fitdistr() uses the "mu" parametrization, i.e.

R> alpha.hat * (1 - p.hat) / p.hat
[1] 0.222497

which roughly matches the ML estimate for mu from fitdistr().
> 2-how can i fit it and than test the goodness of it?
You could use goodfit() from "vcd":

gf_pois <- goodfit(x, type = "poisson")
plot(gf_pois)
summary(gf_pois)

gf_nbin <- goodfit(x, type = "nbinom")
plot(gf_nbin)
summary(gf_nbin)

which shows that the fit from the NB is satisfactory while the fit from 
the Poisson is not.

hth,
Z
> 3-then compare with the Poisson model?
>
> Thanks
>
> ==============================================>
> by negative binomial type two i mean
>
>
>
>
> -- 
> View this message in context:
http://n4.nabble.com/test-the-goodness-of-it-for-negative-binomial-type-2-tp1575892p1575892.html
> Sent from the R help mailing list archive at Nabble.com.
>
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