Likelihood ratios

After posting this, I thought to contact Pete Dixon himself and indeed
it seems he already coded the functions to obtain a likelihood ratio
comparing two lmer models:

AIC_lmer = function(x){
	require(lme4)
	print(formula(attr(x,"call")))
	summary(x)@AICtab
}

LR_lmer = function(m0,m1){
	exp((AIC_lmer(m0)[[1]]-AIC_lmer(m1)[[1]])/2)
}

#example usage:
LR_lmer( my_fit1 , my_fit2 )



On Tue, Jun 1, 2010 at 1:50 PM, Mike Lawrence <Mike.Lawrence at dal.ca> wrote:

oops, I guess that should be:

LR = exp( anova( fit1 , fit2 )$Chisq[2] / -2 )


On Tue, Jun 1, 2010 at 1:28 PM, Mike Lawrence <Mike.Lawrence at dal.ca> wrote:

Hi folks,

I have 2 lmer fits, one (fit1) nested in the other (fit2), and I'd
like to compute the likelihood ratio comparing the models so I can say
something like "there is X times more evidence for fit1 than for fit2"
(as in Glover & Dixon, 2004, www.ncbi.nlm.nih.gov/pubmed/15732688).

I know I can use anova(fit1,fit2) to obtain a null-hypothesis
significance test of the fits, and I suspect the output also contains
the information I need to make my evidentiary statement, but I'm not
confident of what I'm doing here. Is it correct that the reported
value of chi-square from anova() is simply the D of the likelihood
ratio test (http://en.wikipedia.org/wiki/Likelihood_ratio_test)? If
so, does it sound right that I can simply derive the
complexity-corrected likelihood ratio as:

LR = exp( -2 * anova( fit1 , fit2 )$Chisq[2] )

?


Mike

--
Mike Lawrence
Graduate Student
Department of Psychology
Dalhousie University

Looking to arrange a meeting? Check my public calendar:
http://tr.im/mikes_public_calendar

~ Certainty is folly... I think. ~

--
Mike Lawrence
Graduate Student
Department of Psychology
Dalhousie University

Looking to arrange a meeting? Check my public calendar:
http://tr.im/mikes_public_calendar

~ Certainty is folly... I think. ~

--
Mike Lawrence
Graduate Student
Department of Psychology
Dalhousie University

Looking to arrange a meeting? Check my public calendar:
http://tr.im/mikes_public_calendar

~ Certainty is folly... I think. ~