[R-sig-ME] mean and variance of random effects in glmer
Thierry.ONKELINX at inbo.be
Tue Jul 24 21:12:46 CEST 2012
Don't assume that there is not quasi-separation, but rather check it to be sure that it is not there. I'm pretty sure that it is a case of complete separation. Deal with that first.
Van: Ken Kelley [kkelley op nd.edu]
Verzonden: dinsdag 24 juli 2012 20:38
Aan: ONKELINX, Thierry
CC: r-sig-mixed-models op r-project.org
Onderwerp: Re: mean and variance of random effects in glmer
Thanks for your thoughts on this. I hadn't considered quasi-separation, but I don't think that is it. Actually, the issue is that I slipped into thinking that the random effects were the conditional means (like in a linear mixed effects model). Rather, they are the conditional modes. Thus, the mean of the random effects need not be zero as I initially expected (and as would be the case in a linear mixed effects model).
But, I still expected the variance of the random effects to match the output (it is 18.9 in the output yet 7.8 when I calculate it on the random effects directly).
On Jul 24, 2012, at 5:23 AM, "ONKELINX, Thierry" <Thierry.ONKELINX op inbo.be> wrote:
> Dear Ken,
> Very large variance for the random effect in a binomial glmer is an indication for (quasi-)complete separation. Here is some info on that issue: http://www.ats.ucla.edu/stat/mult_pkg/faq/general/complete_separation_logit_models.htm
> If the values of Problem and Across are constant within each level of PID, I would aggregate the data (sum per PID) and then use a simple glm()
> Best regards,
> ir. Thierry Onkelinx
> Instituut voor natuur- en bosonderzoek / Research Institute for Nature and Forest,
> team Biometrie & Kwaliteitszorg / team Biometrics & Quality Assurance
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> Thierry.Onkelinx op inbo.be
> To call in the statistician after the experiment is done may be no more than asking him to perform a post-mortem examination: he may be able to say what the experiment died of.
> ~ Sir Ronald Aylmer Fisher
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> ~ Roger Brinner
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> -----Oorspronkelijk bericht-----
> Van: r-sig-mixed-models-bounces op r-project.org [mailto:r-sig-mixed-models-bounces op r-project.org] Namens Ken Kelley
> Verzonden: dinsdag 24 juli 2012 8:29
> Aan: r-sig-mixed-models op r-project.org
> Onderwerp: [R-sig-ME] mean and variance of random effects in glmer
> Hi everyone,
> I'm fitting a straightforward glmer model with the family=binomial. I expected the mean of the random effect for the intercept to be near zero, but that isn't the case, as the mean is .91:
>> (model.3 <- glmer(TA ~ 1 + Problem + Across + (1|PID), data=Data.Timed, family = binomial, nAGQ=100))
> Generalized linear mixed model fit by the adaptive Gaussian Hermite approximation
> Formula: TA ~ 1 + Problem + Across + (1 | PID)
> Data: Data.Timed
> AIC BIC logLik deviance
> 158.8 172.9 -75.38 150.8
> Random effects:
> Groups Name Variance Std.Dev.
> PID (Intercept) 18.869 4.3439
> Number of obs: 256, groups: PID, 64
> Fixed effects:
> Estimate Std. Error z value Pr(>|z|)
> (Intercept) -1.1328 0.7304 -1.551 0.1209
> Problem -0.5864 0.2449 -2.394 0.0167 *
> Across -1.3768 0.4280 -3.217 0.0013 **
> Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
> Correlation of Fixed Effects:
> (Intr) Problm
> Problem -0.390
> Across -0.208 -0.150
> That is, I'm doing a mixed effects logistic regression. The PID is the participant ID; there are 4 Problems (essentially timepoints: 0, 1, 2, 3) and Across is a time-varying covariate (0, 1, 2, or 3).
> The mean of the random effects is:
> Additionally, the variance of the random effect is in the model output as 18.869, yet when I calculate the variance of the random effects directly, I get a much smaller value:
> (Intercept) 7.806402
> Should I be surprised by either of the issues I note above? My concern is that I was planning on plotting the model implied curves using the fixed effects (so that the curves would represent an individual specific trajectory for a participant with a random effect of 0). Yet, there are no individuals with a random effect of zero and the mean is not zero. Thus, such a plot doesn't seem as useful as I initially thought it would.
> Thanks for any thoughts on this,
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Dit bericht en eventuele bijlagen geven enkel de visie van de schrijver weer en binden het INBO onder geen enkel beding, zolang dit bericht niet bevestigd is door een geldig ondertekend document.
The views expressed in this message and any annex are purely those of the writer and may not be regarded as stating an official position of INBO, as long as the message is not confirmed by a duly signed document.
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