[R-sig-ME] Fw: glmer and nAGQ

[Peter R Law]

Thanks for your response. I'm sorry the formatting of the R output got messed up and hard to read when I forwarded the original message. I do understand that changing the value of nAGQ is changing the approximation of the likelihood, not changing the model itself. I wondered whether the change in likelihood from -643.8 (n=1) to -5.9 (n=2) to -inf (n=3) was a reason to suspect the model was badly behaved in that none of these approximations of the logLik should be trusted. The parameter estimates for the fixed effects don't change that much, basically just the logLik and consequently the AIC. I wanted to compare the glmer with a Gamma distribution to the lmer to see whether AIC favoured one over the other. My intent was to start with the glmer with default n=1, then increase n until model parameters seemed stable, then use that n value for the glmer model to obtain a logLik and then AIC to compare to the lmer version. But the dramatic differences in logLik in going from n=1 to n= 2 and the negative infinite value already for n=3 I thought might suggest that none of these logLiks should be trusted and the the Gamma model simply abandoned. Does that sound plausible in light of the warning messages? In particular, that the negative infinity answer is a failuer in the estimation procedure rather than an indication that that the Gamma model has zero likelihood?

I will repeat the analyses with the log link. As far as the data is concerned, it's simulated data I created without using any distributional assumptions but rather based on ecological assumptions for how the response might depend on the fixed predictors. My purpose here is to explore the glmer function in preparation for a study with real data (still being collected). Because the responses are positive values, I thought it would be worth investigating how a Gamma glmer compares with an lmer (and compare both with a log-normal model, after making the adjustment in the latter so that its AIC is comparable to the others, as was pointed out in a previous posting).

Peter


Sent with ProtonMail Secure Email.

‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐

On Friday, June 18th, 2021 at 5:44 PM, Ben Bolker <bbolker using gmail.com> wrote:

> This is kind of hard to read, but I'll try.
>
> First of all, log-likelihoods with nAGQ>1 (adaptive Gauss-Hermite
>
> quadrature) **are not currently commensurate with log-likelihoods with
>
> nAGQ==1 (Laplace approximation). This is hinted at in the "Deviance and
>
> log-likelihood of GLMMs" section under "Deviance and log-likelihood of
>
> GLMMs".
>
> In any case, the log-likelihoods computed under different numbers of
>
> quadrature points aren't comparable as though they were different
>
> statistical models (AIC, likelihood ratio tests, etc.). Rather, they are
>
> based on different approximations to the same model, and we know
>
> that larger numbers of quadrature points (increasing nAGQ) are more
>
> accurate (although slower) approximations. The main thing is to see if
>
> results (parameter estimates etc.) change as nAGQ increases by a
>
> magnitude that is important for the current analysis ... if they do,
>
> then you should use an nAGQ that is large enough that it approximates
>
> "infinity", i.e. increasing nAGQ further doesn't change the answers by
>
> an important amount.
>
> In general if it is not important to you to use the inverse link (for
>
> interpretation/because you think this will be the natural scale on which
>
> to measure effects of continuous parameters and/or interactions),
>
> Gamma(link="log") is generally more robust.
>
> The "rescale variables?" suggestion is just that; if the parameters
>
> are already rescaled, then it's not going to help. A large eigenvalue
>
> may not actually be a problem, it just indicates the possibility of
>
> numerical instability. In this case my guess is that the very small
>
> random effects variance might be responsible.
>
> It's also a little surprising that the normRain coefficient is very
>
> small in magnitude and yet but has a very large Z-statistic, given this
>
> sample size.
>
> The results with increasing AGQ do indeed look kind of wonky. If this
>
> were my problem I would want to look more carefully at the data/think
>
> about where they came from. How do the model diagnostics look?
>
> On 6/18/21 3:52 PM, Peter R Law via R-sig-mixed-models wrote:
>
> > I am re-sending this query that I originally emailed June 13'th. I did not receive a copy of the sent email as I have with previous postings and my query doesn't appear in the archive of postings so does not seem to have been received.
> >
> > Peter R Law
> >
> > Sent with ProtonMail Secure Email.
> >
> > ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
> >
> > On Monday, June 14th, 2021 at 10:48 PM, Peter R Law prldb using protonmail.com wrote:
> >
> > > Any help with the following query is much appreciated.
> > >
> > > I used some simulated data (not generated under any specific distributional assumption but all responses are positive quantities) to investigate the nAGQ argument in glmer, running a Gamma-distribution model. With nAAGQ=2 the logLik is dramatically different to the default value of nAGQ=1, while nAGQ=5 returned minus infinity for the logLik, but the estimates of the fixed effect parameters are somewhat consistent across each computation. Are the differences in the estimated logLik surprising or do they reflect the warnings glmer returns for this attempted modelling? I got similar results for a real dataset too.
> > >
> > > data.frame':500 obs. of8 variables:
> > >
> > > $ IBI: num25.5 25.4 25.2 25.6 25.8 ...
> > >
> > > $ MatID: Factor w/ 99 levels "M1","M10","M11",..: 1 1 1 1 1 12 12 12 12 12 ...
> > >
> > > $ Pop: Factor w/ 5 levels "P1","P2","P3",..: 1 1 1 1 1 1 1 1 1 1 ...
> > >
> > > $ density : int11 13 15 19 28 11 13 15 19 28 ...
> > >
> > > $ rain: num41.1 36.6 31.6 40 40.6 ...
> > >
> > > $ normIBI : num-1.28 -1.29 -1.31 -1.26 -1.24 ...
> > >
> > > $ normDens: num-1.72 -1.66 -1.61 -1.49 -1.23 ...
> > >
> > > $ normRain: num-0.249 -0.64 -1.073 -0.345 -0.287 ...
> > >
> > > > M61 <- glmer(IBI~normDens+normRain + (1|MatID), family=Gamma, data=Sim)
> > >
> > > Warning message:
> > >
> > > In checkConv(attr(opt, "derivs"), opt$par, ctrl = control$checkConv,:
> > >
> > > Model is nearly unidentifiable: very large eigenvalue
> > >
> > > -   Rescale variables?
> > >
> > > > summary(M61)
> > >
> > > Generalized linear mixed model fit by maximum likelihood (Laplace Approximation) ['glmerMod']
> > >
> > > Family: Gamma( inverse )
> > >
> > > Formula: IBI ~ normDens + normRain + (1 | MatID)
> > >
> > > Data: Sim
> > >
> > > AICBIClogLik deviance df.resid
> > >
> > > 1297.61318.7-643.81287.6495
> > >
> > > Scaled residuals:
> > >
> > > Min1QMedian3QMax
> > >
> > > -1.58054 -0.306190.041690.367441.31012
> > >
> > > Random effects:
> > >
> > > GroupsNameVarianceStd.Dev.
> > >
> > > MatID(Intercept) 5.802e-06 0.002409
> > >
> > > Residual1.368e-03 0.036989
> > >
> > > Number of obs: 500, groups:MatID, 99
> > >
> > > Fixed effects:
> > >
> > > Estimate Std. Error t value Pr(>|z|)
> > >
> > > (Intercept)3.083e-026.090e-0450.63<2e-16 ***
> > >
> > > normDens -1.720e-037.946e-05-21.65<2e-16 ***
> > >
> > > normRain4.889e-042.463e-0519.85<2e-16 ***
> > >
> > > Signif. codes:0 ‘’ 0.001 ‘’ 0.01 ‘’ 0.05 ‘.’ 0.1 ‘ ’ 1
> > >
> > > Correlation of Fixed Effects:
> > >
> > > (Intr) nrmDns
> > >
> > > normDens0.028
> > >
> > > normRain0.000 -0.044
> > >
> > > convergence code: 0
> > >
> > > Model is nearly unidentifiable: very large eigenvalue
> > >
> > > -   Rescale variables?
> > >
> > > What kind of scaling is being suggested in the case of the default value of nAGQ? The predictors are already normalized.
> > >
> > > > M62 <- glmer(IBI~normDens+normRain + (1|MatID), family=Gamma, nAGQ=2,data=Sim)
> > >
> > > boundary (singular) fit: see ?isSingular
> > >
> > > > summary(M62)
> > >
> > > Generalized linear mixed model fit by maximum likelihood (Adaptive Gauss-Hermite Quadrature, nAGQ = 2) ['glmerMod']
> > >
> > > Family: Gamma( inverse )
> > >
> > > Formula: IBI ~ normDens + normRain + (1 | MatID)
> > >
> > > Data: Sim
> > >
> > > AICBIClogLik deviance df.resid
> > >
> > > 21.742.8-5.911.7495
> > >
> > > Scaled residuals:
> > >
> > > Min1QMedian3QMax
> > >
> > > -2.3532 -0.6617 -0.13050.51013.4180
> > >
> > > Random effects:
> > >
> > > GroupsNameVariance Std.Dev.
> > >
> > > MatID(Intercept) 0.000000.0000
> > >
> > > Residual0.024160.1554
> > >
> > > Number of obs: 500, groups:MatID, 99
> > >
> > > Fixed effects:
> > >
> > > Estimate Std. Error t value Pr(>|z|)
> > >
> > > (Intercept)0.02882820.001300822.161< 2e-16 ***
> > >
> > > normDens-0.00392740.0012153-3.2320.00123 **
> > >
> > > normRain0.00027830.00129940.2140.83042
> > >
> > > Signif. codes:0 ‘’ 0.001 ‘’ 0.01 ‘’ 0.05 ‘.’ 0.1 ‘ ’ 1
> > >
> > > Correlation of Fixed Effects:
> > >
> > > (Intr) nrmDns
> > >
> > > normDens -0.275
> > >
> > > normRain0.019 -0.019
> > >
> > > convergence code: 0
> > >
> > > boundary (singular) fit: see ?isSingular
> > >
> > > > M65 <- glmer(IBI~normDens+normRain + (1|MatID), family=Gamma, nAGQ=5,data=Sim)
> > >
> > > > summary(M65)
> > >
> > > Generalized linear mixed model fit by maximum likelihood (Adaptive Gauss-Hermite Quadrature, nAGQ = 5) ['glmerMod']
> > >
> > > Family: Gamma( inverse )
> > >
> > > Formula: IBI ~ normDens + normRain + (1 | MatID)
> > >
> > > Data: Sim
> > >
> > > AICBIClogLik deviance df.resid
> > >
> > > InfInf-InfInf495
> > >
> > > Scaled residuals:
> > >
> > > Min1QMedian3QMax
> > >
> > > -2.2488 -0.39020.03160.34851.7127
> > >
> > > Random effects:
> > >
> > > GroupsNameVarianceStd.Dev.
> > >
> > > MatID(Intercept) 6.007e-06 0.002451
> > >
> > > Residual8.831e-04 0.029716
> > >
> > > Number of obs: 500, groups:MatID, 99
> > >
> > > Fixed effects:
> > >
> > > Estimate Std. Error t value Pr(>|z|)
> > >
> > > (Intercept)2.936e-022.495e-04117.68<2e-16 ***
> > >
> > > normDens-2.182e-031.163e-04-18.76<2e-16 ***
> > >
> > > normRain4.883e-043.973e-0512.29<2e-16 ***
> > >
> > > Signif. codes:0 ‘’ 0.001 ‘’ 0.01 ‘’ 0.05 ‘.’ 0.1 ‘ ’ 1
> > >
> > > Correlation of Fixed Effects:
> > >
> > > (Intr) nrmDns
> > >
> > > normDens -0.013
> > >
> > > normRain0.004 -0.041
> > >
> > > convergence code: 0
> > >
> > > Gradient contains NAs
> > >
> > > Warning messages:
> > >
> > > 1: In vcov.merMod(object, use.hessian = use.hessian) :
> > >
> > > variance-covariance matrix computed from finite-difference Hessian is
> > >
> > > not positive definite or contains NA values: falling back to var-cov estimated from RX
> > >
> > > 2: In vcov.merMod(object, correlation = correlation, sigm = sig) :
> > >
> > > variance-covariance matrix computed from finite-difference Hessian is
> > >
> > > not positive definite or contains NA values: falling back to var-cov estimated from RX
> > >
> > > For comparison, the linear model seems to be well behaved:
> > >
> > > M1 <- lmer(IBI~normDens+normRain +(1|MatID), REML=FALSE, data=Sim)
> > >
> > > summary(M1)
> > >
> > > Linear mixed model fit by maximum likelihood
> > >
> > > ['lmerMod']
> > >
> > > Formula: IBI ~ normDens + normRain + (1 | MatID)
> > >
> > > Data: Sim
> > >
> > > AIC
> > >
> > > BIC
> > >
> > > logLik deviance df.resid
> > >
> > > 1804.2
> > >
> > > 1825.2
> > >
> > > -897.1
> > >
> > > 1794.2
> > >
> > > 495
> > >
> > > Scaled residuals:
> > >
> > > Min
> > >
> > > 1Q
> > >
> > > Median
> > >
> > > 3Q
> > >
> > > Max
> > >
> > > -3.2970 -0.5304
> > >
> > > 0.0153
> > >
> > > 0.5484
> > >
> > > 3.4798
> > >
> > > Random effects:
> > >
> > > Groups
> > >
> > > Name
> > >
> > > Variance Std.Dev.
> > >
> > > MatID
> > >
> > > (Intercept) 43.5694
> > >
> > > 6.6007
> > >
> > > Residual
> > >
> > > 0.6737
> > >
> > > 0.8208
> > >
> > > Number of obs: 500, groups:
> > >
> > > MatID, 99
> > >
> > > Fixed effects:
> > >
> > > Estimate Std. Error t value
> > >
> > > (Intercept) 35.47121
> > >
> > > 0.66443
> > >
> > > 53.39
> > >
> > > normDens
> > >
> > > 2.00732
> > >
> > > 0.11816
> > >
> > > 16.99
> > >
> > > normRain
> > >
> > > -0.71747
> > >
> > > 0.04237
> > >
> > > -16.93
> > >
> > > Correlation of Fixed Effects:
> > >
> > > (Intr) nrmDns
> > >
> > > normDens -0.002
> > >
> > > normRain
> > >
> > > 0.000 -0.044
> > >
> > > Should I just conclude that the data is not well modelled by a Gamma GLMM?
> > >
> > > Peter R Law
> > >
> > > Research Associate
> > >
> > > Center for African Conservation Ecology
> > >
> > > Nelson Mandela University
> > >
> > > South Africa
> > >
> > > Sent with ProtonMail Secure Email.
> > >
> > > [[alternative HTML version deleted]]
> >
> > R-sig-mixed-models using r-project.org mailing list
> >
> > https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models
>
> R-sig-mixed-models using r-project.org mailing list
>
> https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models