If it helps: the compareML() help says that "model comparison is only
implemented for the methods GCV, fREML, REML, and ML."
(so GCV seems possible.)
In the following vignette, it is additionally stated that:
"For testing the difference in fixed effects predictors the method
fREML does not provide the most reliable test. Rather use ML. However,
ML takes longer to run and penalizes wigglyness more."
https://cran.r-project.org/web/packages/itsadug/vignettes/test.html
Jo?o
On Tue, 2020-02-04 at 13:30 +0100, Cesko Voeten wrote:
Hi David,
I've never used compareML myself, but after taking a quick look at
the code it seems that it's just doing a likelihood-ratio test, which
means that the same caveats apply. So if my understanding of this
function is correct, it can be used with gam models fitted using
outer iteration with method="ML" or method="REML" (which are based on
penalized (restricted) maximum likelihood), but not with gam models
fitted using GCV/UBRE/Cp (which are not likelihood-based), gam models
fitted using non-default optimizers (these use PQL), or bam models
(PQL). What I mean by this is that, while the numerical comparison of
the optimized log-likelihood values may very well work for selecting
the best model, the penalized quasilikelihood is not a true
likelihood and hence cannot fall back on Wilks's theorem that the
difference in log-likelihoods is chi-square-distributed. The
analogous reasoning goes for AIC and BIC. But that doesn't mean that
such comparisons are useless -- remember that all models are wrong,
but some are useful.
Note that the only thing select=TRUE does is enable additional
penalization of the smooths; it is up to you to determine how much
shrinkage you deem enough to warrant removing a term from the model.
So if you feel that your one main effect is important to you, you can
always choose to leave it in. However, if you go this route, might I
suggest taking a look at the parsimonious mixed models paper (
https://arxiv.org/abs/1506.04967), where the bottom line is: if you
have reasons to expect a term to be important or unimportant, why
even bother with selection procedures and why not just fit the model
that you believe represents your data the best? (In fact, I
personally would only use stepwise-selection methods if I wasn't sure
whether a term is or is not important, particularly with respect to
achieving convergence...)
Best,
Cesko
Op 4-2-2020 om 12:30 schreef David Villegas R?os:
Thanks Cesko,
I also tried the /compareML/ function (itsadug library) to compare
1) the model with all terms included (the best model according to
"buildbam") and 2) the model with one main effect out (the best
according to select=TRUE procedure) and /compareML/ supports the
full model (same as buildbam) when both models 1) and 2) are fit
using select=FALSE.
Just in case you have experience with /compareML /too:)
David
El mar., 4 feb. 2020 a las 11:27, Cesko Voeten (<
c.c.voeten at hum.leidenuniv.nl <mailto:c.c.voeten at hum.leidenuniv.nl>>
) escribi?:
Hi David,
I wrote that package, so yes, I am very familiar with it :) :)
buildbam is based on the explained deviance. A term is included
if the explained deviance with the term is higher than the
explained deviance without it. This means that buildbam will favor
models that contain (too?) many effects, as effects that model only
noise but still explain perhaps 0.0001% more deviance due to chance
will still be included. To be honest with you, I have come to
believe that the buildbam function was a mistake in the first
place. I had coded buildgam already and buildbam was an easy
extension, and it was only later that I realized that bam using PQL
meant that only the explained deviance would be a valid model-
comparison criterion. But note that the explained deviance is not
actually a _formal_ criterion, like the likelihood-ratio test is
(which uses a well-known result that differences in log-likelihood
are asymptotically chi-square-distributed) or like AIC or BIC
(which are respectively based on information theory and on Bayesian
inference). I had
wanted to remove buildbam for a long time, but this would be
wrong of me to do because now that it's out there people may be
using it, and given the provided caveat that it is can only use the
explained deviance, they may actually have a use case for it. Your
message makes me think very seriously about officially deprecating
this function in buildmer's next release, or at least issue a
warning that explained deviance is not a formal criterion and will
favor overfitted models. In fact, I should deprecate that as well
-- I had only put it in for buildbam's use.
If buildbam and bam(select=TRUE) conflict, I would definitely
trust bam(select=TRUE) over buildbam, as mgcv's automatic
smoothness selection is a method derived from first principles by
extending the approach used to fit smooth terms anyway in a very
straightforward way. On the other hand, the explained deviance is
completely ad hoc and has no formal justification. I'll just bite
the bullet and deprecate buildbam, advising people to use buildgam
or select=TRUE instead.
Thanks for helping me bite that bullet,
Cesko
Op 4-2-2020 om 10:51 schreef David Villegas R?os:
> Thanks Cesko.
> I tried your suggestion (using the select argument) and got
to a reasonable optimal model in which I keep all the main effects
and interactions except for one main effect.
> I tried, however, to do model selection using the buildbam
function (buildmer library) and I get a different optimal model,
where all effects are highly significant. I?m not sure why.
According to the buildmer package documentation:
>
> "As bam uses PQL, only crit='deviance' is supported."
> where
> crit=Character string or vector determining the criterion
used to test terms for elimination. Possible options are 'LRT'
(likelihood-ratio test; this is the default), 'LL' (use the raw -2
log likelihood), 'AIC' (Akaike Information Criterion), 'BIC'
(Bayesian Information Criterion), and 'deviance' (explained
deviance ? note that this is not a formal test)
>
> I?m not sure if you are familiar with buildmer package
>
> Thanks,
>
> David
>
> El lun., 3 feb. 2020 a las 13:09, Voeten, C.C. (<
c.c.voeten at hum.leidenuniv.nl <mailto:c.c.voeten at hum.leidenuniv.nl>
<mailto:c.c.voeten at hum.leidenuniv.nl <mailto:
c.c.voeten at hum.leidenuniv.nl>>>) escribi?:
>
> Hi David,
>
> Please keep the mailing list in cc.
>
> Yes, gam() can indeed be prohibitively slow, hence why I
suggested the select=TRUE approach. I hope the below explanation
helps.
>
> Smooth terms are composed of a null space and a range
space, which respectively represent the completely smooth part of
the effect and the wiggly part of the effect. In the mathematical
representation, these are equivalent to fixed effects and random
effects, respectively. As such, the range space is subject to
penalization and can shrink to zero. In GAM terms, this would
correspond to a smoothing parameter tending to infinity, resulting
in a completely smooth effect: you would be left only with the null
space. What select=TRUE does is add an additional penalty to all
null spaces, so that these too can shrink towards zero. Effects
which are shrunk to (near) zero in this way can then be removed
from the model by the user.
>
> The degree of shrinkage can be seen by looking at the
edf. Smooths that are (near) zero will also use (near) zero edf. In
practice, when I do model selection using select=TRUE, I always
remove terms whose edf < 1. Then I fit the model again with
select=TRUE to ensure that no further reduction is necessary, and
then I fit a final model with select=FALSE.
>
> Best,
> Cesko
>
> -----Oorspronkelijk bericht-----
> Van: David Villegas R?os <chirleu at gmail.com <mailto:
chirleu at gmail.com> <mailto:chirleu at gmail.com <mailto:
chirleu at gmail.com>>>
> Verzonden: maandag 3 februari 2020 11:28
> Aan: Voeten, C.C. <c.c.voeten at hum.leidenuniv.nl <mailto:
c.c.voeten at hum.leidenuniv.nl> <mailto:c.c.voeten at hum.leidenuniv.nl
<mailto:c.c.voeten at hum.leidenuniv.nl>>>
> Onderwerp: Re: [R-sig-ME] bam model selection with 3
>
> Thanks Cesko, really appreciate your answer.
> In my particular case however, I cannot run gam models
(they take forever to run) so I?m still struggling on how to
perform model selection in bam. I tried select=TRUE and the summary
changed a bit, but I don?t really know what this argument is doing
behind the scenes, or whether I should trust the summary from the
model using select=TRUE rather than the default select=FALSE.
> Best wishes,
> David
>
> El s?b., 1 feb. 2020 a las 20:39, Voeten, C.C. (<mailto:
c.c.voeten at hum.leidenuniv.nl <mailto:c.c.voeten at hum.leidenuniv.nl>
<mailto:c.c.voeten at hum.leidenuniv.nl <mailto:
c.c.voeten at hum.leidenuniv.nl>>>) escribi?:
> Hi David,
>
> 1) You cannot perform likelihood-based model comparisons
with bam models, or -- for completeness' sake -- with gam models
that were fitted using performance iteration or the EFS optimizer.
All of these are based on PQL (penalized quasi-likelihood), which
makes the log-likelihood (and hence LRT, AIC, BIC, etc) invalid for
comparison purposes. See Wood (2017:149-151). gam() with the
default outer iteration should be fine, though. Have you tried
fitting your full model using bam with the select=TRUE argument to
turn on mgcv's automatic smooth-term selection?
>
> 2) I am unsure if the deviance explained is or is not
suitable for indicating effect size, so I can't comment on this
question. I might, however, have an alternative suggestion: have
you considered partial eta squared or partial omega squared? You
should be able to calculate those based on the ANOVA table.
>
> 3) I agree with you that the warning suggests complete
separation, but in my experience this doesn't automatically have to
be a problem. Have you checked the summary for extremely large beta
values, and also have you run gam.check() to see if your fit looks
reasonable? If neither indicates a problem I wouldn't be too
concerned about it.
>
> Hope this helps,
>
> Cesko
>
> P.S.: please send messages in plain text only, as you
can see the formatting of your message was slightly screwed up
because the mailing list automatically strips HTML markup
>
> -----Oorspronkelijk bericht-----
> Van: R-sig-mixed-models <mailto:
r-sig-mixed-models-bounces at r-project.org <mailto:
r-sig-mixed-models-bounces at r-project.org> <mailto:
r-sig-mixed-models-bounces at r-project.org <mailto:
r-sig-mixed-models-bounces at r-project.org>>> Namens David Villegas
R?os
> Verzonden: zaterdag 1 februari 2020 19:57
> Aan: r-sig-mixed-models <mailto:
r-sig-mixed-models at r-project.org <mailto:
r-sig-mixed-models at r-project.org> <mailto:
r-sig-mixed-models at r-project.org <mailto:
r-sig-mixed-models at r-project.org>>>
> Onderwerp: [R-sig-ME] bam model selection with 3 million
>
> Dear list,
>
> I?m investigating the effect of three variables (X, Y,
Z) on the probability that an animal uses a particular habitat A. I
have a time series of relocations for each animal (>300
individuals), with one relocation every 30 minutes. There are only
two options for the response
> variable: 1=present in habitat A, 0=not present in
habitat A. The effects of the three variables are expected to be
non-linear so I?m using gam models. My dataset is very large, with
3 million data points so I?m using the bam function from the mgcv
library in R. In my models I include a random effect ?individual
ID?, and a temporal autocorrelation term that corrects much but not
all of the autocorrelation in the models.
>
> *Question 1.*
>
> When I run a model with the three main effects (X, Y, Z)
and the three double interactions (X:Y, X:Z, Y:Z), I get that all
terms are highly significant, except for one interaction. If I
remove it, then everything is highly significant. However, I also
wanted to run simpler models with only one interaction, no
interactions, only two main effects and only one main effect. Then,
if I compare all these models with AIC or BIC, I get that the best
model (by far) is the one with only main effects.
AIC(codcoaAR2,codcoaAR2.1,codcoaAR2.2,codcoaAR2.3,codcoaAR2.4,codco
aAR2.5,codcoaAR2.6,codcoaAR2.7,codcoaAR2.8,codcoaAR2.9,codcoaAR)
>
> df AIC
>
> codcoaAR2 306.1310 -1442543
>
> codcoaAR2.1 293.1608 -1440642
>
> codcoaAR2.2 292.9615 -1438219
>
> codcoaAR2.3 294.3657 -1435346
>
> codcoaAR2.4 284.0026 -1434286
>
> codcoaAR2.5 280.3472 -1396765
>
> codcoaAR2.6 279.6380 -1435862
>
> codcoaAR2.7 269.4968 -1377806
>
> codcoaAR2.8 269.0480 -1393897
>
> codcoaAR2.9 281.8584 -1214270
>
> codcoaAR 271.7066 -2353481 # model with only main
>
>
>
> I wonder how this is possible if two of the interactions
>
> So my underlying question is: *for a model like this in
which sample size is huge, should I make model selection looking at
the significance of the different terms in the model, or should I
rather look at AIC/BIC?*
>
> *Question 2.*
>
> Let?s assume the model with only main effects is indeed
> Then I?d like to get the effect size of each explanatory
variable. It?s not clear to me how to do it even after reading some
post on this and other forums, but I tried to figure it out by
sequentially running the model without one explanatory variable at
a time, and then comparing the deviance explained in the optimal
model with X, Y, Z with the deviance explained with the reduced
model with only Y and Z, for instance. Assuming that the difference
would the variance explained by X. *Is this correct? *Looking at
the results, the deviance explained by each variable X, Y, Z is
quite low, but if the three main effects explain so little
variance, who is explaining the rest?
>
> Model
>
> Deviance explained
>
> X, Z, Y
>
> 69.3%
>
> Y, Z
>
> 68.5%
>
> X, Z
>
> 69.3%
>
> X, Y
>
> 60.5%
>
>
>
> *Question 3.*
>
> In my models I usually get this error message:
>
> Warning message:
>
> In bgam.fitd(G, mf, gp, scale, nobs.extra = 0, rho =
>
> fitted probabilities numerically 0 or 1 occurred
>
>
>
> which seems to indicate that there is perfect separation
in my logistic regression. I?m not sure this is the case in my
data, how could I check it and correct for it if needed? Should it
be always corrected?
>
>
>
> Thanks for your help,
>
> David
>
> [[alternative HTML version deleted]]
>
> _______________________________________________
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