[R-sig-ME] testing fixed effects in binomial lmer...again?

Dimitris Rizopoulos dimitris.rizopoulos at med.kuleuven.be
Tue Jan 8 18:13:47 CET 2008

----- Original Message ----- 
From: "Douglas Bates" <bates at stat.wisc.edu>
To: "Achaz von Hardenberg" <fauna at pngp.it>
Cc: <r-sig-mixed-models at r-project.org>
Sent: Tuesday, January 08, 2008 3:10 PM
Subject: Re: [R-sig-ME] testing fixed effects in binomial 

> On Jan 8, 2008 5:38 AM, Achaz von Hardenberg <fauna at pngp.it> wrote:
>> Dear all,
>> I know that this may be a already debated topic, but even searching
>> the R-help and the r-sig-mixed-models archives I can not find a 
>> reply
>> to my doubts...(but see Ben Bolkers' reply to my similar quest in 
>> r-
>> help).
>> I am performing a binomial glmm analysis using the lmer function in
>> the lme4 package (last release, just downloaded). I am using the
>> "Laplace method".
> Yes, that is the best choice in lmer.  (In the development version 
> it
> is, in fact, the only choice.)
>> However, I am not sure about what I should do to test for the
>> significance of fixed effects in the binomial case: Is it correct 
>> to
>> test a full model against a model from which I remove the fixed
>> effect I want to test using the anova(mod1.lmer, mod2.lmer) method
>> and then relying on the model with the lower AIC (or on the Log-
>> likelihood test?)?
> The change in the log-likelihood between two nested models is, in my
> opinion, the most sensible test statistic for comparing the models.
> However, it is not clear how one should convert this test statistic 
> to
> a p-value.  The use of the chi-squared distribution is based on
> asymptotic results and can give an "anti-conservative" (i.e. lower
> than would be obtained through a randomization test or via 
> simulation)
> p-value for small samples.  As far as I can see, the justification 
> for
> the use of AIC as a comparison criterion is even more vague.
> For linear fixed-effects models one can compensate for small samples
> by changing from z-tests to t-tests and from chi-squared tests to F
> tests.  The exact theory breaks down for mixed-effects models or for
> generalized linear models and is even more questionable for
> generalized linear mixed models.  As Ben Bolker mentioned, I think
> that one way to deal with the hypothesis testing question while
> preserving the integrity of the model is to base inferences on a
> Markov-chain Monte Carlo sample from the (Bayesian) posterior
> distribution of the parameters.
> Code for MCMC samples for parameters in GLMMs is not yet fully
> developed (or documented).  In the meantime I would use the 
> likelihood
> ratio tests but exercise caution in reporting p-values for
> small-sample cases.

What about Bootstrap (parametric or not)? Would it be useful in this 

(For instance, something along the following lines:


form.null <- # formula under null
form.altr <- # formula under alternative
fm1 <- lmer(form.null, family = binomial, data = data)
fm2 <- lmer(form.altr, family = binomial, data = data)

# observed value of the LRT
Tobs <- anova(fm1, fm2)$Chisq[2]

B <- 199
Tvals <- numeric(B)
# 'id' is the grouping variable
unq.ids <- unique(data$id)
for (b in 1:B) {
    dat.new <- # a sample with replacement from the original subjects
    fm1 <- lmer(form.null, family = binomial, data = data.new)
    fm2 <- lmer(form.altr, family = binomial, data = data.new)
    Tvals[b] <- anova(fm1, fm2)$Chisq[2]
# estimated p-value
(1 + sum(Tvals >= Tobs)) / (B + 1)

if the estimated p-value is near the significance level, 'B' can be 
increased accordingly.)


>> Would you advice me to use the glmmML function instead? (I am not
>> sure where the differences are with lmer)
>> I thank in advance for your help!
>> best regards,
>> Achaz von Hardenberg
>> Ben Bolker wrote:
>>  >The short answer is that testing fixed effects in GLMMs
>>  >is difficult and dangerous.  Likelihood ratio tests on fixed
>>  >effect differences [which is generally what anova() does]
>>  >in a random-effects model are unreliable
>>  >(see Pinheiro and Bates 2000).  Most of the world does
>>  >F tests with various corrections on the denominator
>>  >degrees of freedom, but this is contentious (in particular,
>>  >Doug Bates, the author of lme4, disagrees).  lme4 will
>>  >eventually let you use an MCMC sampling method to test
>>  >fixed effects but that may or may not be working
>>  >in the current version.
>>  >I would try this question again on the r-sig-mixed
>>  >mailing list.
>>  > good luck,
>>  > Ben Bolker
>> Dr. Achaz von Hardenberg
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