gam {mgcv} | R Documentation |
Fits the specified generalized additive model (GAM) to
data. gam()
is not a clone of what Splus provides.
Smooth terms are represented using penalized regression splines
with smoothing parameters selected by GCV or by regression splines with
fixed degrees of freedom (mixtures of the two are
permitted). Multi-dimensional smooths are available using penalized thin plate
regression splines, but the user must make sure that covariates are sensibly scaled
relative to each other when using such terms. For a general overview see
Wood (2001).
gam(formula,family=gaussian(),data=list(),weights=NULL,control=gam.control,scale=0)
formula |
A GAM formula. This is exactly like the formula for a
glm except that smooth terms can be added to the right hand side of the
formula (and a formula of the form y ~ . is not allowed).
Smooth terms are specified by expressions of the form:
s(var1,var2,...,k=12,fx=FALSE,bs="tp") where var1 ,
var2 , etc. are the covariates which the smooth
is a function of and k is the dimension of the basis used to
represent the smooth term. If k is not
specified then k=10*3^(d-1) is used where d is the number
of covariates for this term. fx is used to indicate whether or
not this term has a fixed muber of degrees of freedom (fx=FALSE
to select d.f. by GCV/UBRE). bs indicates the basis to use, with
"cr" indicating cubic regression spline, and "tp"
indicating thin plate regression spline: "cr" can only be used
with 1-d smooths.
For backwards compatibility the formula may also include terms like s(x,12|f) , which specifies a regression spline which is not to be penalized
and has 12 knots, or s(x,z,25) indicating a rank 25 penalized
t.p.r.s. In such cases arguements k , fx and bs are
ignored if supplied and a one dimensional term will always use a cubic
regression spline basis. Note that a term of the form s(x) will
result in a term with a "tp" basis. |
family |
This is a family object specifying the distribution and link to use in
fitting etc. See glm and family for more
details. Where the family is neg.binom then a negative binomial
family is used based on the implementation in the MASS library.
In this case, if the value of theta is not given, a version of
glm.nb :gam.nbut is used to estimate theta iteratively, starting from a
Poisson distribution. This extra layer of iteration slows down fitting.
|
data |
A data frame containing the model response variable and covariates required by the
formula. If this is missing then the frame from which gam was called is
searched for the variables specified in the formula. |
weights |
prior weights on the data. |
control |
A list as returned by gam.control , with five user controllable elements:
maxit controls maximum iterations in
gam.fit , convergence tolerance in gam.fit is controlled by epsilon
and the third item is trace . The smoothing
parameter selection method is controlled by two further
items: mgcv.tol controls the convergence tolerance
to use in smoothing parameter estimation, while
mgcv.half.max controls the maximum number of step
halvings to try in each optimization step if the step fails to
reduce the GCV score. |
scale |
If this is zero then GCV is used for all distributions
except Poisson, binomial and negative binomial where UBRE is used with scale parameter
assumed to be 1. If this is greater than 1 it is assumed to be the scale
parameter/variance and UBRE is used. If scale is negative GCV
is always used (for binomial models in particular, it is probably worth
comparing UBRE and GCV results; for ``over-dispersed Poisson'' GCV is
probably more appropriate than UBRE.) |
Two alternative bases are available for representing model terms. Univariate smooth terms can be represented using conventional cubic regression splines - which are very efficient computationally - or thin plate regression splines. Multivariate terms must be represented using thin plate regression splines. For either basis the user specifies the dimension of the basis for each smooth term. The dimension of the basis is one more than the maximum degrees of freedom that the term can have, but usually the term will be fitted by penalized maximum likelihood estimation and the actual degrees of freedom will be chosen by GCV. However, the user can choose to fix the degrees of freedom of a term, in which case the actual degrees of freedom will be one less than the basis dimension.
Thin plate regression splines are constructed by starting with the basis for a full thin plate spline and then truncating this basis in an optimal manner, to obtain a low rank smoother. Details are given in Wood (MS submitted). One key advantage of the approach is that it avoids the knot placement problems of conventional regression spline modelling, but it also has the advantage that smooths of lower rank are nested within smooths of higher rank, so that it is legitimate to use conventional hypothesis testing methods to compare models based on pure regression splines.
In the case of the cubic regression spline basis, knots of the spline are placed evenly
throughout the covariate values to which the term refers: For
example, if fitting 101 data with an 11 knot spline of x
then
there would be a knot at every 10th (ordered) x
value. The
parameterization used represents the spline in terms of its
values at the knots. Connection of these values at neighbouring knots
by sections of cubic polynomial constrainted to join at the knots so as to be
continuous up to and including second derivative yields a natural cubic
spline through the values at the knots (given two extra conditions specifying
that the second derivative of the curve should be zero at the two end
knots). This parameterization gives the parameters a nice interpretability.
Given a basis for each smooth term, it easy to obtain a wiggliness penalty for each, and to construct a penalized likelihood, which balances the fit of the overall model against it's complexity. This consists of the log likelihood for the model minus a sum of wiggliness penalties (one for each smooth) each multiplied by a smoothing parameter. The smoothing parameters control the trade-off between fit and smoothness.
So, the gam fitting problem has become a penalized glm fitting problem, which can be fitted using a
slight modification of glm.fit
: gam.fit
. The penalized
glm approach also allows smoothing parameters for all smooth terms to
be selected simultaneously by GCV or UBRE. This is achieved as
part of fitting by calling mgcv
within gam.fit
.
Details of the GCV/UBRE minimization method are given in Wood (2000).
The function returns an object of class "gam"
which has the following elements:
coefficients |
the coefficients of the fitted model. Parametric coefficients are first, followed by coefficients for each spline term in turn. |
residuals |
the deviance residuals for the fitted model. |
fitted.values |
fitted model predictions of expected value for each datum. |
family |
family object specifying distribution and link used. |
linear.predictor |
fitted model prediction of link function of expected value for each datum. |
deviance |
(unpenalized) |
null.deviance |
deviance for single parameter model. |
df.null |
null degrees of freedom |
iter |
number of iterations of IRLS taken to get convergence. |
weights |
final weights used in IRLS iteration. |
prior.weights |
prior weights on observations. |
df.null |
number of data |
y |
response data. |
converged |
indicates whether or not the iterative fitting method converged. |
sig2 |
estimated or supplied variance/scale parameter. |
edf |
estimated degrees of freedom for each smooth. |
boundary |
did parameters end up at boundary of parameter space? |
sp |
smoothing parameter for each smooth. |
df |
number of knots for each smooth (one more than maximum degrees of freedom). |
nsdf |
number of parametric, non-smooth, model terms including the intercept. |
Vp |
estimated covariance matrix for parameter estimators. |
covariate.shift |
covariates get shifted so that they are centred around zero - this is by how much. |
xp |
knot locations for each cubic regression spline based smooth.
xp[i,] are the locations for the ith smooth. |
UZ |
array storing the matrices for transforming from t.p.r.s. basis to equivalent t.p.s. basis - see GAMsetup for details of how the matrices are packed in this array. |
Xu |
The set of unique covariate locations used to define t.p.s. from which t.p.r.s. basis was derived. Again see GAMsetup for details of the packing algorithm. |
xu.length |
The number of unique covariate combinations in the data. |
formula |
the model formula. |
full.formula |
the model formula with each smooth term fully expanded and with option arguments given explicitly (i.e. not with reference to other variables) - useful for later prediction from the model. |
x |
parametric design matrix columns (including intercept) followed by the data that form arguments of the smooths. |
s.type |
type of spline basis used: 0 for conventional cubic regression spline, 1 for t.p.r.s. |
p.order |
the order of the penalty used for each term. 0 signals auto-selection. |
dim |
number of covariates of which term is a function |
call |
a mode call object containing the call to gam() that produced
this gam object (useful for constructing model frames). |
mgcv.conv |
A list of smoothing parameter convergence diagnostics, with the following elements (irrelevant for models with only one smoothing parameter to estimate):
g above - i.e. the leading diagonal of the Hessian.TRUE if the second smoothing parameter guess improved the GCV/UBRE score. (Please report examples
where this is FALSE )TRUE if the algorithm terminated by failing to improve the GCV/UBRE score rather than by "converging".
Not necessarily a problem, but check the above derivative information quite carefully. |
The code does not check for rank defficiency of the model matrix -it will likely just fail instead!
You must have more unique combinations of covariates than the model has total parameters. (Total parameters is sum of basis dimensions plus sum of non-spline terms less the number of spline terms).
Automatic smoothing parameter selection is not likely to work well when fitting models to very few response data.
Relative scaling of covariates to a multi-dimensional smooth term has an affect on the results: make sure that relative scalings are sensible. For example, measuring one spatial co-ordinate in millimetres and the other in lightyears will usually produce poor results.
With large datasets (more than a few thousand data) the "tp"
basis gets very slow to use. In this case use "cr"
for 1-d
smooths. If you need to use multi-dimensional terms with large datasets
and find gam
too slow, please let me know - and I'll up the
priority for fixing this!
Simon N. Wood snw@st-and.ac.uk
Hastie and Tibshirani (1990) Generalized Additive Models. Chapman and Hall.
Green and Silverman (1994) Nonparametric Regression and Generalized Linear Models. Chapman and Hall.
Gu and Wahba (1991) Minimizing GCV/GML scores with multiple smoothing parameters via the Newton method. SIAM J. Sci. Statist. Comput. 12:383-398
Wood (2000) Modelling and Smoothing Parameter Estimation with Multiple Quadratic Penalties. JRSSB 62(2):413-428
Wood (2001) mgcv:GAMs and Generalized Ridge Regression for R. R News 1(2):20-25
Wood (MS submitted) Thin Plate Regression Splines
Wahba (1990) Spline Models of Observational Data. SIAM
http://www.ruwpa.st-and.ac.uk/simon.html
library(mgcv) set.seed(1) n<-400 sig2<-4 x0 <- runif(n, 0, 1) x1 <- runif(n, 0, 1) x2 <- runif(n, 0, 1) x3 <- runif(n, 0, 1) pi <- asin(1) * 2 f <- 2 * sin(pi * x0) f <- f + exp(2 * x1) - 3.75887 f <- f + 0.2 * x2^11 * (10 * (1 - x2))^6 + 10 * (10 * x2)^3 * (1 - x2)^10 - 1.396 e <- rnorm(n, 0, sqrt(abs(sig2))) y <- f + e b<-gam(y~s(x0)+s(x1)+s(x2)+s(x3)) plot(b,pages=1) # now a GAM with 3df regression spline term & 2 penalized terms b1<-gam(y~s(x0,k=4,fx=TRUE,bs="tp")+s(x1,k=12)+s(x2,15)) plot(b1,pages=1) # now fit a 2-d term to x0,x1 b3<-gam(y~s(x0,x1)+s(x2)+s(x3)) par(mfrow=c(2,2)) plot(b3) par(mfrow=c(1,1)) # now simulate poisson data g<-exp(f/5) y<-rpois(rep(1,n),g) b2<-gam(y~s(x0)+s(x1)+s(x2)+s(x3),family=poisson) plot(b2,pages=1) # negative binomial data set.seed(1) y<-rnbinom(g,size=2,mu=g) b3<-gam(y~s(x0)+s(x1)+s(x2)+s(x3),family=neg.binom) plot(b3,pages=1) # and a pretty 2-d smoothing example.... test1<-function(x,z,sx=0.3,sz=0.4) { (pi**sx*sz)*(1.2*exp(-(x-0.2)^2/sx^2-(z-0.3)^2/sz^2)+ 0.8*exp(-(x-0.7)^2/sx^2-(z-0.8)^2/sz^2)) } n<-500 old.par<-par(mfrow=c(2,2)) x<-runif(n);z<-runif(n); xs<-seq(0,1,length=30);zs<-seq(0,1,length=30) pr<-data.frame(x=rep(xs,30),z=rep(zs,rep(30,30))) truth<-matrix(test1(pr$x,pr$z),30,30) contour(xs,zs,truth) y<-test1(x,z)+rnorm(n)*0.1 b4<-gam(y~s(x,z)) fit1<-matrix(predict.gam(b4,pr,se=FALSE),30,30) contour(xs,zs,fit1) persp(xs,zs,truth) persp(b4) par(old.par)