Re: [R] ML fit of gamma distribution to grouped data

From: <Augusto.Sanabria_at_ga.gov.au>
Date: Thu 30 Nov 2006 - 00:09:30 GMT


Thomas,

The Gamma distr. can be fitted via ML using:

Library("MASS")

GF <- fitdistr(given_data,"gamma")
sh <- GF$estimate[1]
ra <- GF$estimate[2]

Fitting via Moments, m:
var <- m[2] - m[1]*m[1]

sh <- m[1]*m[1]/var
sc <- m[1]/var
ra <- 1/sc

G_pdf <- dgamma(breaks,shape=sh,rate=ra,scale=1/ra)

Hope it helps,

Augusto



Augusto Sanabria. MSc, PhD.
Mathematical Modeller
Risk Research Group
Geospatial & Earth Monitoring Division
Geoscience Australia (www.ga.gov.au)
Cnr. Jerrabomberra Av. & Hindmarsh Dr.
Symonston ACT 2601
Ph. (02) 6249-9155    

-----Original Message-----
From: r-help-bounces@stat.math.ethz.ch
[mailto:r-help-bounces@stat.math.ethz.ch] On Behalf Of Thomas Petzoldt Sent: Tuesday, 28 November 2006 10:26
To: r-help@stat.math.ethz.ch
Subject: [R] ML fit of gamma distribution to grouped data

Hello,

we have a set of biological cell-size data, which are only available as frequencies of discrete size classes, because of the high effort of manual microscopic measurements.

The lengths are approximately gamma distributed, however the shape of the distribution is relatively variable between different samples (maybe it's a mixture in reality).

Is there any ML fitting (or moment-based) procedure for the gamma distribution and grouped data already available in R?

Here is a small example:

breaks <- c(0, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150)
mids   <- c(10, 25, 35, 45, 55, 65, 75, 85, 95, 125)
counts <- c(87, 5, 2, 2, 1, 1, 0, 0, 1, 1)

Any help is highly appreciated

Thomas P.



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R-help@stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Thu Nov 30 15:35:22 2006

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