Date:         Sun, 17 Jan 2010 19:12:28 -0800
Reply-To:     Ryan <ryan.andrew.black@GMAIL.COM>
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         Ryan <ryan.andrew.black@GMAIL.COM>
Organization: http://groups.google.com
Subject:      Re: model comparison
Comments: To: sas-l@uga.edu
Content-Type: text/plain; charset=ISO-8859-1

On Jan 17, 2:29 am, Ching <kcwo...@gmail.com> wrote:
> Hi everyone.
>
> I've done some reading about halving the p-value (difference in -2LL)
> when comparing 2 different models produced by proc mixed. Is it more
> appropriate this way? but why?
>
> Your advice is greatly appreciated. Have a great day.

Ching,

To be clear, the difference in -2 log likelihoods approximates a Chi-
Square distribution with degrees of freedom equal to the difference in
the # of estimated parameters for each model. The assumption of this
test is that one model is nested within the other model. In response
to your question, I have never heard of halving the p-value associated
with this Chi-Square test.

Ryan