[R-sig-ME] lmer vs glmmPQL
Douglas Bates
bates at stat.wisc.edu
Thu Jun 25 00:48:08 CEST 2009
On Wed, Jun 24, 2009 at 4:51 PM, Ben Bolker<bolker at ufl.edu> wrote:
> Roger Levy wrote:
>> Ben Bolker wrote:
>>> Daniel Ezra Johnson wrote:
>>>> Regarding this thread, what about the method of fitting nested models
>>>> and using anova() to estimate a p-value.
>>>>
>>>> For example:
>>>>
>>>> mod2 = lmer(y ~ genotype + (1|block), family = binomial, data)
>>>> mod0 = lmer(y ~ (1|block), family = binomial, data)
>>>>
>>>> anova(mod0,mod2)
>>>>
>>>> How does that p-value (which is one value for the whole term
>>>> "genotype") relate to the individual coefficient p-values derived from
>>>> the Z-scores inside summary(mod2)?
>>>>
>>>> Thanks,
>>>> Dan
>>> The Z-scores are Wald tests. The anova results are likelihood
>>> ratio tests. The latter are in general more reliable but are known
>>> to be *un*reliable for small-sample (i.e.
>>> small-number-of-random-effects-levels) LMMs (Pinheiro and Bates).
>>> Wald tests are not *known* to be unreliable in the small-sample
>>> case, but I believe that is a statement of ignorance rather than
>>> a statement that they're OK ...
>>
>> This is an interesting issue -- as Ben points out, likelihood-ratio
>> tests are known to be anti-conservative for tests between linear mixed
>> models differing only in fixed-effects structure. I think the
>> best-known demonstration of this can be found in Pinheiro & Bates, 2000,
>> pages 87-92. F-tests are less problematic.
>
> But note that F tests may not be available or well-defined for
> GLMMs ... (Distinguishing here between conditional F tests (e.g. p. 90
> of Pinheiro & Bates 2000) and Wald F tests of parameter combinations --
> as far as I know the former are not applicable to GLMMs.
> Wald F tests are always available but don't necessarily (???) share
> the same properties/reliability as the conditional F tests in the LMM
> case ...)
>
>> For logit mixed models, presumably the results will depend a bit on how
>> fitting is done because the likelihood has to be approximated. For the
>> Laplace approximation currently in use in lme4, I've done some
>> simulations and generally found that likelihood-ratio tests are still
>> anti-conservative; tests based on Wald Z are not as anti-conservative,
>> though they don't seem quite nominal.
>
> Hmmm. Can't we separate estimation (for which Laplace and AGQ are
> probably adequate) from inference? Would there be any point in
> considering inference on estimates from a less accurate (PQL)
> approximation scheme as long as we have more accurate schemes available?
>
> I worry whether the Wald Z tests are consistently better or
> just "differently wrong" ...
>
> You can do simulations along
>> these lines:
>>
>> set.seed(2)
>> invlogit <- function(x) {
>> temp <- exp(x)
>> return(temp/(1+temp))
>> }
>> L <- 10
>> REP <- 4
>> N <- 4*10
>> fac <- gl(L,REP)
>> f <- function(ignore) {
>> x <- runif(N)
>> r <- rnorm(L)
>> y <- rbinom(N,1,invlogit(r[fac]))
>> m0 <- lmer(y ~ (1 | fac), family="binomial")
>> m1 <- lmer(y ~ x + (1 | fac), family="binomial")
>> temp <- pnorm(fixef(m1)[2]/sqrt(vcov(m1)[2,2]))
>> p.z <- 2*min(temp, 1-temp)
>> p.lr <- 1-pchisq(as.numeric(2*(logLik(m1)-logLik(m0))),1)
>> return(c(p.z,p.lr))
>> }
>> K <- 5000
>> p.vals <- sapply(1:K,f)
>> apply(p.vals, 1, function(x) sum(x<0.05))/K
>> # this gave me 0.0562 for Wald Z, 0.0608 for LRT
>>
>> # Is Wald Z anti-conservative? The below says "yes" with p=0.044
>> 2*(1-pbinom(sum(p.vals[1,]<0.05), K, 0.05))
>>
>>
>> Unlike what Ben states above, however, it's my intuition (and informal
>> observation on the basis of simulations) that what determines how
>> anticonservative the LRT is is not the number of random effect levels
>> but rather the ratio between the degrees of freedom involved in the LRT
>> to the residual degrees of freedom in the more general model. The more
>> residual degrees of freedom (basically, the more observations), the less
>> anti-conservative the LRT is. This can be assessed for a specific
>> dataset by fitting a null-hypothesis model, repeatedly generating new,
>> artificial responses from it, and looking at the distribution of LRTs on
>> the artificial responses (along the lines of P&B 2000).
>
> Your conclusion agrees with what Pinheiro and Bates say (p. 88:
> "as the number of parameters being removed from the fixed effects
> becomes large, compared to the total number of observations, this
> inaccuracy in the reported p-values can be substantial").
> I don't really know how to decide what is more relevant to
> "asymptopia" -- if N is the total number of obs., n the number
> of blocks (say m = number per block so N=mn), and p the number of
> parameters (fixed? random? variances counting as 1, or n-1, or
> something in between?), is it 1-p/N [as PB suggest], or (N-p) [suggested
> above], or n? Demidenko p. 145 shows that for LMMs N \to \infty is
> not sufficient for consistency, that n \to \infty is also required ...
> I don't know if that helps in practice.
>
> I don't know where my intuition comes from -- perhaps it's just
> conservatism/pessimism, since it's often easier to get N>>p than to get
> n large ...
>
> I do agree that the gold standard is simulations as suggested above.
> There are examples in the "worked example" section of glmm.wikidot.com .
> These simulations can be very slow -- hopefully (??) Doug Bates will be
> able to work out an mcmcsamp() scheme for GLMMs soon, and we can see
> if that (much faster) approach generally agrees with null simulations ...
If you look at Doug's schedule for July (www.stat.wisc.edu/~bates) you
will understand why he has been missing in action recently. In my
"spare time" - when not working on a major report finished last week
and a grant proposal due this week and in preparing these various
courses - I am working on the draft of a book about lme4 because if I
don't get some part of that manuscript to John Kimmel soon he is going
to be very rude to me in Rennes.
>
>
> @book{demidenko_mixed_2004,
> edition = {1},
> title = {Mixed Models: Theory and Applications},
> isbn = {0471601616},
> publisher = {{Wiley-Interscience}},
> author = {Eugene Demidenko},
> month = jul,
> year = {2004}
> }
>
>
>>
>> Roger
>>
>>
>
>
> --
> Ben Bolker
> Associate professor, Biology Dep't, Univ. of Florida
> bolker at ufl.edu / www.zoology.ufl.edu/bolker
> GPG key: www.zoology.ufl.edu/bolker/benbolker-publickey.asc
>
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