[R-sig-ME] Random vs. fixed effects
Ben Bolker
bolker at ufl.edu
Fri Apr 23 20:11:45 CEST 2010
Here's my question for the group:
Given that it is a reasonable *philosophical* position to say 'treat
philosophically random effects as random no matter what, and leave them
in the model even if they don't appear to be statistically significant',
and given that with small numbers of random-effect levels this approach
is likely to lead to numerical difficulties in most (??) mixed model
packages (warnings, errors, or low estimates of the variance), what
should one do? (Suppose one is in a situation that is too complicated
to use classical method-of-moments approaches -- crossed designs, highly
unbalanced data, GLMMs ...)
1. philosophy, schmilosophy. Fit these factors as a fixed effect,
anything else is too dangerous/misleading/unworkable.
2. proceed with the 'standard' mixed model (lme4, nlme, PROC MIXED,
...) and hope it doesn't break. Ignore warnings.
3. use Bayesian-computational approaches (MCMCglmm, WinBUGS, AD Model
Builder with post-hoc MCMC calculation? Data cloning?)? Possibly with
half-Cauchy priors on variance as recommended by Gelman [Bayesian
Analysis (2006) 1, Number 3, pp. 515–533]?
Gabor Grothendieck wrote:
> Here is a simulation of 10k cases using 4 and 50 level factors for the
> random effect. With 4 levels there are numerical problems and the
> accuracy of the random effect is terrible. With 50 levels there are
> no numerical problems and the accuracy is much better.
>
>> library(lme4)
>> set.seed(1)
>> n <- 10000
>> k <- 4
>> f <- function(n, k) {
> + set.seed(1)
> + x <- 1:n
> + fac <- gl(k, 1, n)
> + fac.eff <- rnorm(k, 0, 4)[fac]
> + e <- rnorm(n)
> + y <- 1 + 2 * x + fac.eff + e
> + lmer(y ~ x + (1|fac))
> + }
>
>> # simulation with 4 level random effect
>> f(n, 4)
> Linear mixed model fit by REML
> Formula: y ~ x + (1 | fac)
> AIC BIC logLik deviance REMLdev
> 28733 28762 -14363 28702 28725
> Random effects:
> Groups Name Variance Std.Dev.
> fac (Intercept) 1.1162 1.0565
> Residual 1.0298 1.0148
> Number of obs: 10000, groups: fac, 4
>
> Fixed effects:
> Estimate Std. Error t value
> (Intercept) 1.313e+00 5.286e-01 2
> x 2.000e+00 3.515e-06 568923
>
> Correlation of Fixed Effects:
> (Intr)
> x -0.033
> Warning message:
> In mer_finalize(ans) : false convergence (8)
>
>> # simulation with 50 level random effect
>> f(n, 50)
> Linear mixed model fit by REML
> Formula: y ~ x + (1 | fac)
> AIC BIC logLik deviance REMLdev
> 29040 29069 -14516 29009 29032
> Random effects:
> Groups Name Variance Std.Dev.
> fac (Intercept) 11.2016 3.3469
> Residual 1.0251 1.0125
> Number of obs: 10000, groups: fac, 50
>
> Fixed effects:
> Estimate Std. Error t value
> (Intercept) 1.396e+00 4.738e-01 3
> x 2.000e+00 3.507e-06 570242
>
> Correlation of Fixed Effects:
> (Intr)
> x -0.037
>
>
>
>
> On Fri, Apr 23, 2010 at 9:38 AM, Schultz, Mark R. <Mark.Schultz2 at va.gov> wrote:
>> I just read a post by Andrew Dolman suggesting that a factor with only 3
>> levels should be treated as a fixed effect. This seems to be a perennial
>> question with mixed models. I'd really like to hear opinions from
>> several experts as to whether there is a consensus on the topic. It
>> really makes me uncomfortable that such an important modeling decision
>> is made with an "ad hoc" heuristic.
>>
>>
>>
>> Thanks,
>>
>> Mark Schultz, Ph.D.
>>
>> Bedford VA Hospital
>>
>> Bedford, Ma.
>>
>>
>> [[alternative HTML version deleted]]
>>
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>>
>
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--
Ben Bolker
Associate professor, Biology Dep't, Univ. of Florida
bolker at ufl.edu / people.biology.ufl.edu/bolker
GPG key: people.biology.ufl.edu/bolker/benbolker-publickey.asc
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