Additive versus multiplicative overdispersion modeling

David,

To somewhat wrap things up; as I guess would be expected, I get the same
repeatability estimate from a quasibinomial model using V/(V+sigma^2*pi^2/3)
as with V/(V+pi^2/3) from a binomial model.

Thanks again for your and everyone else's help!
Ned

--
Ned Dochtermann
Department of Biology
University of Nevada, Reno

ned.dochtermann at gmail.com
http://wolfweb.unr.edu/homepage/mpeacock/Dochter/
--



-----Original Message-----
From: David Duffy [mailto:davidD at qimr.edu.au] 
Sent: Sunday, August 22, 2010 6:39 PM
To: Ned Dochtermann
Cc: r-sig-mixed-models at r-project.org
Subject: Re: [R-sig-ME] Additive versus multiplicative overdispersion
modeling

Thanks a lot, if that is indeed the case it makes calculating
repeatabilities per N&S quite straightforward for the multiplicative
models (quasibinomial & quasipoisson) since the relevant term to
include in the denominator would just be (summary(model)@sigma)^2
(multiplied by (pi^2)/3 ). Of course I still can't figure out how to
get the needed information from the additive models, i.e. the residual
of the distribution specific variance.

Method "C" in the Browne paper uses: r = V/(V+pi^2/3) for the logistic 
link, and r=V/(V+1) for the probit link (the latter is the tetrachoric r).

Cheers, David Duffy.

| David Duffy (MBBS PhD)                                         ,-_|\
| email: davidD at qimr.edu.au  ph: INT+61+7+3362-0217 fax: -0101  /     *
| Epidemiology Unit, Queensland Institute of Medical Research   \_,-._/
| 300 Herston Rd, Brisbane, Queensland 4029, Australia  GPG 4D0B994A v