[R] A model-building strategy in mixed-effects modelling

[Ben Bolker]

Stats Wolf <stats.wolf <at> gmail.com> writes:

> 
> Dear all,
> 
> Consider a completely randomized block design (let's use data(Oats)
> irrespoctive of the split-plot design it was arranged in). Look:
> 
> library(nlme)
> fit <- lme(yield ~ nitro, Oats, random = ~1|Block, method="ML")
> fit2 <- lm(yield ~ nitro + Block, Oats)
> anova(fit, fit2)
> gives this:
>      Model df      AIC      BIC    logLik   Test  L.Ratio p-value
> fit      1  4 624.3245 633.4312 -308.1623
> fit2     2  8 611.9309 630.1442 -297.9654 1 vs 2 20.39366   4e-04
> 
> Clearly, considering block a random term is worse than considering it
> a fixed term. 

  I have technical concerns with this step (and larger
philosophical concerns with the whole approach -- see below).
It's not at all clear that anova() applied to these two models
makes sense -- in fact I think it doesn't, because the two
models aren't nested. (I guess technically I could take the
limit as the random effects variance goes to infinity,
or 1/variance -> 0, to get from the random to the fixed
specification.  Then the LRT would be suspect on the grounds
that the null hypothesis (1/variance=0) is on the boundary of
the allowable region, but that will typically "only" bias the
p-value by a factor of 2 ...)  Furthermore, I'm not 100% convinced
that the likelihoods computed by lm() and lme() are comparable,
constant terms are often left out.
  But let's suppose this comparison is OK ...


Let's see if blocking should be included in the model at
> all:
> 
> fit3 <- lm(yield ~nitro, Oats)
> anova(fit2,fit3)
> 
> which gives a very small P value in favor of fit2, which suggests the
> block term should be included. So, I go for the second model, with
> block considered fixed.
> 
> Is this indeed how I should generally proceed when choosing the
> optimum model for a situation that calls for mixed effects? Of course,
> the example above is overly simplistic, yet such situations can occur

  I think in general that you should decide on "random" vs "fixed"
on philosophical and practical grounds, _a priori_ ... if you're going
to be Bayesian (see various discussions by Andrew Gelman on the subject)
then you don't have any philosophical problems at all, because "fixed"
and "pooled" are just two ends of a spectrum (variance of the priors
of the effects fixed at infinity and zero, respectively), and it's
just a practical question of estimation.  Doing a lot of hypothesis
testing to decide on the best model starts down the road of
data-dredging ...

  In general, questions like this might get more attention
on r-sig-mixed-models at lists.r-project.org

  cheers
    Ben Bolker