[R] negative binomial overdispersion question
kartik@pdx.edu
kartik at pdx.edu
Wed Nov 23 01:28:05 CET 2005
Hello,
I'm a grad student in the Intelligent Transportation Systems lab at Portland
State Univ. in Portland, OR, USA. I'm trying to learn the basics of R to run a
negative binomial in the near future, and so I ran a test regression on roadway
crash data obtained from "Statistical and Econometric Methods for
Transportation Data Analysis" by Washington et al (p. 250). I ran the test
(glm.nb from library MASS) and got the same output as in the text for all the
parameters except the overdispersion parameter; the text lists 0.516, but R
gave me an overdispersion parameter output of 1.9365. (I've attached the
raw dataset, in Excel .csv format, if anyone wants to try the test. If the
attachment does not go through, please email me and I'll forward it on to you.)
Has anyone had similar problems with the overdispersion parameter output from R?
Any thoughts on this would be much appreciated.
Thanks!
Kartik
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