[R-sig-ME] [R] negative variances
Douglas Bates
bates at stat.wisc.edu
Sat Apr 14 17:33:46 CEST 2007
On 4/12/07, Tu Yu-Kang <yukangtu at hotmail.com> wrote:
> Dear Prof Bates,
>
> > lm2<-lmer(ppd~month+(month|id))
> Warning message:
> Estimated variance-covariance for factor 'id' is singular
> in: `LMEoptimize<-`(`*tmp*`, value = list(maxIter = 200, tolerance =
> 1.49011611938477e-08,
> > lm2<-lmer(ppd~month+(month|id),control = list(msVerbose = TRUE) )
> 0 1191.73: 0.888889 0.0592593 0.00000
> 1 1174.58: 5.00000e-10 5.00000e-10 0.281419
> 2 1158.10: 0.956617 0.00924174 -0.00978256
> 3 1151.88: 0.875101 5.00000e-10 0.550741
> 4 1144.22: 0.605105 5.00000e-10 0.550741
> 5 1142.59: 0.335108 5.00000e-10 0.550741
> 6 1142.06: 0.452561 5.00000e-10 0.550741
> 7 1141.97: 0.423318 5.00000e-10 0.550740
> 8 1141.97: 0.416524 5.00000e-10 0.550740
> 9 1141.97: 0.417039 5.00000e-10 0.550740
> Warning message:
> Estimated variance-covariance for factor 'id' is singular
> in: `LMEoptimize<-`(`*tmp*`, value = list(maxIter = 200, tolerance =
> 1.49011611938477e-08,
> > lm3<-lmer2(ppd~month+(month|id),control = list(msVerbose = TRUE) )
> 0 1191.73: 0.942809 0.243432 0.00000
> 1 1181.40: 0.870948 0.00000 -0.124082
> 2 1146.91: 0.855545 2.18615e-05 0.0163376
> 3 1140.17: 0.413096 0.00000 0.0798584
> 4 1137.48: 0.484553 2.13939e-09 0.179021
> 5 1136.23: 0.542748 0.00000 0.0799814
> 6 1135.87: 0.508893 0.0117775 0.110609
> 7 1135.79: 0.496375 0.00000 0.115248
> 8 1135.79: 0.497595 0.00000 0.117998
> 9 1135.79: 0.494810 2.92361e-08 0.119135
> 10 1135.79: 0.494925 1.82196e-08 0.119321
> 11 1135.79: 0.495102 0.00000 0.119390
> 12 1135.79: 0.495102 0.00000 0.119390
> >
Both of those optimization paths show demonstrate convergence to a
singular variance-covariance matrix. The first two parameters being
optimized are on the scale of variances (lmer) or standard deviations
(lmer2) and are constrained to be non-negative. In the case of lmer
they need to be constrained to be slightly positive (>= 1e-10). You
can see that both case have converged on the boundary although lmer2
achieved a better minimum (lower deviance).
The first parameter represents the relative variance (or relative
standard deviation for lmer2) of the intercept random effect. The
second parameter is a relative variance (standard deviation) of a
linear combination of the random effects. The third parameter
determines the linear combination.
To me this indicates model failure. There are finite variances for
both the slope and intercept random effects but they are perfectly
correlated.
> The -2loglikelihhood from MLwiN is 1104.594. I did plot the data and they
> looked fine (at least to me).
>
> With best regards,
>
> Yu-Kang
>
>
>
>
> >From: "Douglas Bates" <bates at stat.wisc.edu>
> >To: "Tu Yu-Kang" <yukangtu at hotmail.com>
> >CC: r-sig-mixed-models at r-project.org
> >Subject: Re: [R] negative variances
> >Date: Thu, 12 Apr 2007 08:07:57 -0500
> >
> >On 4/12/07, Tu Yu-Kang <yukangtu at hotmail.com> wrote:
> >>
> >>Dear Prof Bates,
> >>
> >>Many thanks for your email. I tried lmer() and received the
> >>following
> >>messages:
> >>
> >> > lm2<-lmer(ppd~month+(month|id))
> >>Warning message:
> >>Estimated variance-covariance for factor 'id' is singular
> >> in: `LMEoptimize<-`(`*tmp*`, value = list(maxIter = 200,
> >>tolerance =
> >>1.49011611938477e-08,
> >>
> >>I then tried lmer2():
> >>
> >> > lm3<-lmer2(ppd~month+(month|id))
> >> > summary(lm3)
> >>Linear mixed-effects model fit by REML
> >> AIC BIC logLik MLdeviance REMLdeviance
> >> 1146 1166 -567.9 1126 1136
> >>Random effects:
> >> Groups Name Variance Std.Dev. Corr
> >> id (Intercept) 0.1566631 0.395807
> >> month 0.0022331 0.047255 1.000
> >> Residual 0.6391120 0.799445
> >>Number of obs: 420, groups: id, 140
> >>
> >>Fixed effects:
> >> Estimate Std. Error t value
> >>(Intercept) 6.43595 0.07017 91.72
> >>month -0.36619 0.01642 -22.30
> >>
> >>Correlation of Fixed Effects:
> >> (Intr)
> >>month -0.544
> >
> >I'm a bit pressed for time right now so this will be brief. I
> >suggest
> >that you add the optional argument
> >
> >control = list(msVerbose = TRUE)
> >
> >to the calls to lmer and to lmer2 and look at the progress of the
> >iterations and also at the value of the deviance. I have seen
> >situations where lmer2 converges to a fit with a significantly
> >smaller
> >deviance than can lmer because it progresses through the
> >near-singular
> >region of the parameter space.
> >
> >What was the deviance (or the log-likelihood) from the MLWin fit
> >that
> >gave negative variance components?
> >
> >Did you plot the data as ppd versus month by id using xyplot from
> >the
> >lattice package? That should always be the first step in the
> >analysis
> >of longitudinal data.
> >
> >>However, I am not sure about the results, because MLwiN showed both
> >>random
> >>effects were negative values (-0.196 and -0.023).
> >>
> >>I start to notice this problems of negative variances when I am
> >>learning
> >>how to use structural equation modeling software to run multilevel
> >>models
> >>for longitudinal data. To my great surprise, it occurs quite
> >>frequently. In
> >>SEM, this problem sometimes may be overcome by estimating a
> >>nonlinear
> >>model by freeing the factor loadings. For example, in this data,
> >>PPD
> >>(probing pocket depth) was measured three times at month 0, 3 and
> >>6. I only
> >>fixed the first and last factor loadings to be 0 and 6 to get a
> >>non-linear
> >>relation, and I also allow the level-1 residuals to be different on
> >>each
> >>occasion. However, in some data, I failed to get a satifactory
> >>model no
> >>matter how I modified my models.
> >>
> >>I looked for the discussion in several multilevel modeling
> >>textbooks but
> >>only found one short discussion in the book by Brown and Prescott.
> >>SEM
> >>literature usually suggest fixing the negative variances to 0.
> >>However, I
> >>wander whether this is the only way to get around this problem or
> >>the
> >>sensible way because if the random effects are fixed to 0 the model
> >>is no
> >>longer a random effects model.
> >>
> >>With best regards,
> >>
> >>Yu-Kang
>
> _________________________________________________________________
> 立即申請 MSN Mobile 服務：用手機和 MSN Messenger 網友隨時交談
> http://msn.com.tw/msnmobile
>
>
More information about the R-sig-mixed-models
mailing list