jagam {mgcv}  R Documentation 
Just Another Gibbs Additive Modeller: JAGS support for mgcv.
Description
Facilities to autogenerate model specification code and associated data to simulate with GAMs in JAGS (or BUGS). This is useful for inference about models with complex random effects structure best coded in JAGS. It is a very innefficient approach to making inferences about standard GAMs. The idea is that jagam
generates template JAGS code, and associated data, for the smooth part of the model. This template is then directly edited to include other stochastic components. After simulation with the resulting model, facilities are provided for plotting and prediction with the model smooth components.
Usage
jagam(formula,family=gaussian,data=list(),file,weights=NULL,na.action,
offset=NULL,knots=NULL,sp=NULL,drop.unused.levels=TRUE,
control=gam.control(),centred=TRUE,sp.prior = "gamma",diagonalize=FALSE)
sim2jam(sam,pregam,edf.type=2,burnin=0)
Arguments
formula 
A GAM formula (see 
family 
This is a family object specifying the distribution and link function to use.
See 
data 
A data frame or list containing the model response variable and
covariates required by the formula. By default the variables are taken
from 
file 
Name of the file to which JAGS model specification code should be written. See 
weights 
prior weights on the data. 
na.action 
a function which indicates what should happen when the data contain ‘NA’s. The default is set by the ‘na.action’ setting of ‘options’, and is ‘na.fail’ if that is unset. The “factoryfresh” default is ‘na.omit’. 
offset 
Can be used to supply a model offset for use in fitting. Note
that this offset will always be completely ignored when predicting, unlike an offset
included in 
control 
A list of fit control parameters to replace defaults returned by

knots 
this is an optional list containing user specified knot values to be used for basis construction.
For most bases the user simply supplies the knots to be used, which must match up with the 
sp 
A vector of smoothing parameters can be provided here.
Smoothing parameters must be supplied in the order that the smooth terms appear in the model
formula (without forgetting null space penalties). Negative elements indicate that the parameter should be estimated, and hence a mixture
of fixed and estimated parameters is possible. If smooths share smoothing parameters then 
drop.unused.levels 
by default unused levels are dropped from factors before fitting. For some smooths involving factor variables you might want to turn this off. Only do so if you know what you are doing. 
centred 
Should centring constraints be applied to the smooths, as is usual with GAMS? Only set
this to 
sp.prior 

diagonalize 
Should smooths be reparameterized to have i.i.d. Gaussian priors (where possible)? For Gaussian data this
allows efficient conjugate samplers to be used, and it can also work well with GLMs if the JAGS 
sam 
jags sample object, containing at least fields 
pregam 
standard 
edf.type 
Since EDF is not uniquely defined and may be affected by the stochastic structure added to the JAGS model file, 3 options are offered. See details. 
burnin 
the amount of burn in to discard from the simulation chains. Limited to .9 of the chain length. 
Details
Smooths are easily incorportated into JAGS models using multivariate normal priors on the smooth coefficients. The smoothing parameters and smoothing penalty matrices directly specifiy the prior multivariate normal precision matrix. Normally a smoothing penalty does not correspond to a full rank precision matrix, implying an improper prior inappropriate for Gibbs sampling. To rectify this problem the null space penalties suggested in Marra and Wood (2011) are added to the usual penalties.
In an additive modelling context it is usual to centre the smooths, to avoid the identifiability issues associated with having an intercept for each smooth term (in addition to a global intercept). Under Gibbs sampling with JAGS it is technically possible to omit this centring, since we anyway force propriety on the priors, and this propiety implies formal model identifiability. However, in most situations this formal identifiability is rather artificial and does not imply statistically meaningfull identifiability. Rather it serves only to massively inflate confidence intervals, since the multiple intercept terms are not identifiable from the data, but only from the prior. By default then, jagam
imposes standard GAM identifiability constraints on all smooths. The centred
argument does allow you to turn this off, but it is not recommended. If you do set centred=FALSE
then chain convergence and mixing checks should be particularly stringent.
The final technical issue for model setup is the setting of initial conditions for the coefficients and smoothing parameters. The approach taken is to take the default initial smoothing parameter values used elsewhere by mgcv
, and to take a single PIRLS fitting step with these smoothing parameters in order to obtain starting values for the smooth coefficients. In the setting of fully conjugate updating the initial values of the coefficients are not critical, and good results are obtained without supplying them. But in the usual setting in which slice sampling is required for at least some of the updates then very poor results can sometimes be obtained without initial values, as the sampler simply fails to find the region of the posterior mode.
The sim2jam
function takes the partial gam
object (pregam
) from jagam
along with simulation output in standard rjags
form and creates a reduced version of a gam
object, suitable for plotting and prediction of the model's smooth components. sim2gam
computes effective degrees of freedom for each smooth, but it should be noted that there are several possibilites for doing this in the context of a model with a complex random effects structure. The simplest approach (edf.type=0
) is to compute the degrees of freedom that the smooth would have had if it had been part of an unweighted Gaussian additive model. One might choose to use this option if the model has been modified so that the response distribution and/or link are not those that were specified to jagam
. The second option is (edf.type=1
) uses the edf that would have been computed by gam
had it produced these estimates  in the context in which the JAGS model modifications have all been about modifying the random effects structure, this is equivalent to simply setting all the random effects to zero for the effective degrees of freedom calculation. The default option (edf.type=2
) is to base the EDF on the sample covariance matrix, Vp
, of the model coefficients. If the simulation output (sim
) includes a mu
field, then this will be used to form the weight matrix W
in XWX = t(X)%*%W%*%X
, where the EDF is computed from rowSums(Vp*XWX)*scale
. If mu
is not supplied then it is estimated from the the model matrix X
and the mean of the simulated coefficients, but the resulting W
may not be strictly comaptible with the Vp
matrix in this case. In the situation in which the fitted model is very different in structure from the regression model of the template produced by jagam
then the default option may make no sense, and indeed it may be best to use option 0.
Value
For jagam
a three item list containing
pregam 
standard 
jags.data 
list of arguments to be supplied to JAGS containing information referenced in model specification. 
jags.ini 
initialization data for smooth coefficients and smoothing parameters. 
For sim2jam
an object of class "jam"
: a partial version of an mgcv
gamObject
, suitable for
plotting and predicting.
WARNINGS
Gibb's sampling is a very slow inferential method for standard GAMs. It is only likely to be worthwhile when complex random effects structures are required above what is possible with direct GAMM methods.
Check that the parameters of the priors on the parameters are fit for your purpose.
Author(s)
Simon N. Wood simon.wood@rproject.org
References
Wood, S.N. (2016) Just Another Gibbs Additive Modeller: Interfacing JAGS and mgcv. Journal of Statistical Software 75(7):115 doi:10.18637/jss.v075.i07)
Marra, G. and S.N. Wood (2011) Practical variable selection for generalized additive models. Computational Statistics & Data Analysis 55(7): 23722387
Here is a key early reference to smoothing using BUGS (although the approach and smooths used are a bit different to jagam)
Crainiceanu, C. M. D Ruppert, & M.P. Wand (2005) Bayesian Analysis for Penalized Spline Regression Using WinBUGS Journal of Statistical Software 14.
See Also
Examples
## the following illustrates a typical workflow. To run the
## 'Not run' code you need rjags (and JAGS) to be installed.
require(mgcv)
set.seed(2) ## simulate some data...
n < 400
dat < gamSim(1,n=n,dist="normal",scale=2)
## regular gam fit for comparison...
b0 < gam(y~s(x0)+s(x1) + s(x2)+s(x3),data=dat,method="REML")
## Set directory and file name for file containing jags code.
## In real use you would *never* use tempdir() for this. It is
## only done here to keep CRAN happy, and avoid any chance of
## an accidental overwrite. Instead you would use
## setwd() to set an appropriate working directory in which
## to write the file, and just set the file name to what you
## want to call it (e.g. "test.jags" here).
jags.file < paste(tempdir(),"/test.jags",sep="")
## Set up JAGS code and data. In this one might want to diagonalize
## to use conjugate samplers. Usually call 'setwd' first, to set
## directory in which model file ("test.jags") will be written.
jd < jagam(y~s(x0)+s(x1)+s(x2)+s(x3),data=dat,file=jags.file,
sp.prior="gamma",diagonalize=TRUE)
## In normal use the model in "test.jags" would now be edited to add
## the nonstandard stochastic elements that require use of JAGS....
## Not run:
require(rjags)
load.module("glm") ## improved samplers for GLMs often worth loading
jm <jags.model(jags.file,data=jd$jags.data,inits=jd$jags.ini,n.chains=1)
list.samplers(jm)
sam < jags.samples(jm,c("b","rho","scale"),n.iter=10000,thin=10)
jam < sim2jam(sam,jd$pregam)
plot(jam,pages=1)
jam
pd < data.frame(x0=c(.5,.6),x1=c(.4,.2),x2=c(.8,.4),x3=c(.1,.1))
fv < predict(jam,newdata=pd)
## and some minimal checking...
require(coda)
effectiveSize(as.mcmc.list(sam$b))
## End(Not run)
## a gamma example...
set.seed(1); n < 400
dat < gamSim(1,n=n,dist="normal",scale=2)
scale < .5; Ey < exp(dat$f/2)
dat$y < rgamma(n,shape=1/scale,scale=Ey*scale)
jd < jagam(y~s(x0)+te(x1,x2)+s(x3),data=dat,family=Gamma(link=log),
file=jags.file,sp.prior="log.uniform")
## In normal use the model in "test.jags" would now be edited to add
## the nonstandard stochastic elements that require use of JAGS....
## Not run:
require(rjags)
## following sets random seed, but note that under JAGS 3.4 many
## models are still not fully repeatable (JAGS 4 should fix this)
jd$jags.ini$.RNG.name < "base::MersenneTwister" ## setting RNG
jd$jags.ini$.RNG.seed < 6 ## how to set RNG seed
jm <jags.model(jags.file,data=jd$jags.data,inits=jd$jags.ini,n.chains=1)
list.samplers(jm)
sam < jags.samples(jm,c("b","rho","scale","mu"),n.iter=10000,thin=10)
jam < sim2jam(sam,jd$pregam)
plot(jam,pages=1)
jam
pd < data.frame(x0=c(.5,.6),x1=c(.4,.2),x2=c(.8,.4),x3=c(.1,.1))
fv < predict(jam,newdata=pd)
## End(Not run)