rq.Rd
\name{rq}
\alias{rq}
\title{
Quantile Regression
}
\description{
Returns an object of class \code{"rq"} \code{"rqs"}
or \code{"rq.process"} that represents a quantile regression fit.
}
\usage{
rq(formula, tau=.5, data, subset, weights, na.action,
method="br", model = TRUE, contrasts, \dots)
}
\arguments{
\item{formula}{
a formula object, with the response on the left of a \code{~} operator,
and the terms, separated by \code{+} operators, on the right.
}
\item{tau}{
the quantile(s) to be estimated, this is generally a number strictly between 0 and 1,
but if specified strictly outside this range, it is presumed that the solutions
for all values of \code{tau} in (0,1) are desired. In the former case an
object of class \code{"rq"} is returned, in the latter,
an object of class \code{"rq.process"} is returned. As of version 3.50,
tau can also be a vector of values between 0 and 1; in this case an
object of class \code{"rqs"} is returned containing among other things
a matrix of coefficient estimates at the specified quantiles.
}
\item{data}{
a data.frame in which to interpret the variables
named in the formula, or in the subset and the weights argument.
If this is missing, then the variables in the formula should be on the
search list. This may also be a single number to handle some special
cases -- see below for details.
}
\item{subset}{
an optional vector specifying a subset of observations to be
used in the fitting process.}
\item{weights}{
vector of observation weights; if supplied, the algorithm fits
to minimize the sum of the weights multiplied into the
absolute residuals. The length of weights must be the same as
the number of observations. The weights must be nonnegative
and it is strongly recommended that they be strictly positive,
since zero weights are ambiguous.
}
\item{na.action}{
a function to filter missing data.
This is applied to the model.frame after any subset argument has been used.
The default (with \code{na.fail}) is to create an error if any missing values are
found. A possible alternative is \code{na.omit}, which
deletes observations that contain one or more missing values.
}
\item{model}{if TRUE then the model frame is returned. This is
essential if one wants to call summary subsequently.
}
\item{method}{
the algorithmic method used to compute the fit. There are several
options: The default method is the modified version of the
Barrodale and Roberts algorithm for \eqn{l_1}{l1}-regression,
used by \code{l1fit} in S, and is described in detail in
Koenker and d'Orey(1987, 1994), default = \code{"br"}.
This is quite efficient for problems up to several thousand observations,
and may be used to compute the full quantile regression process. It
also implements a scheme for computing confidence intervals for
the estimated parameters, based on inversion of a rank test described
in Koenker(1994). For larger problems it is advantageous to use
the Frisch--Newton interior point method \code{"fn"}.
And very large problems one can use the Frisch--Newton approach after
preprocessing \code{"pfn"}. Both of the latter methods are
described in detail in Portnoy and Koenker(1997).
There is a fifth option \code{"fnc"} that enables the user to specify
linear inequality constraints on the fitted coefficients; in this
case one needs to specify the matrix \code{R} and the vector \code{r}
representing the constraints in the form \eqn{Rb \geq r}. See the
examples. Finally, there are two penalized methods: \code{"lasso"}
and \code{"scad"} that implement the lasso penalty and Fan and Li's
smoothly clipped absolute deviation penalty, respectively. These
methods should probably be regarded as experimental.
}
\item{contrasts}{
a list giving contrasts for some or all of the factors
default = \code{NULL} appearing in the model formula.
The elements of the list should have the same name as the variable
and should be either a contrast matrix (specifically, any full-rank
matrix with as many rows as there are levels in the factor),
or else a function to compute such a matrix given the number of levels.
}
\item{...}{
additional arguments for the fitting routines
(see \code{\link{rq.fit.br}} and \code{\link{rq.fit.fnb}}
and the functions they call).
}
}
\value{
See \code{\link{rq.object}} and \code{\link{rq.process.object}} for details.
Inferential matters are handled with \code{\link{summary}}. There are
extractor methods \code{logLik} and \code{AIC} that are potentially
relevant for model selection.
}
\details{For further details see the vignette available from \R with
\code{ vignette("rq",package="quantreg")} and/or the Koenker (2005).
For estimation of nonlinear (in parameters) quantile regression models
there is the function \code{nlrq} and for nonparametric additive
quantile regression there is the function \code{rqss}.
Fitting of quantile regression models with censored data is handled by the
\code{crq} function.}
\examples{
data(stackloss)
rq(stack.loss ~ stack.x,.5) #median (l1) regression fit for the stackloss data.
rq(stack.loss ~ stack.x,.25) #the 1st quartile,
#note that 8 of the 21 points lie exactly on this plane in 4-space!
rq(stack.loss ~ stack.x, tau=-1) #this returns the full rq process
rq(rnorm(50) ~ 1, ci=FALSE) #ordinary sample median --no rank inversion ci
rq(rnorm(50) ~ 1, weights=runif(50),ci=FALSE) #weighted sample median
#plot of engel data and some rq lines see KB(1982) for references to data
data(engel)
attach(engel)
plot(income,foodexp,xlab="Household Income",ylab="Food Expenditure",type = "n", cex=.5)
points(income,foodexp,cex=.5,col="blue")
taus <- c(.05,.1,.25,.75,.9,.95)
xx <- seq(min(income),max(income),100)
f <- coef(rq((foodexp)~(income),tau=taus))
yy <- cbind(1,xx)\%*\%f
for(i in 1:length(taus)){
lines(xx,yy[,i],col = "gray")
}
abline(lm(foodexp ~ income),col="red",lty = 2)
abline(rq(foodexp ~ income), col="blue")
legend(3000,500,c("mean (LSE) fit", "median (LAE) fit"),
col = c("red","blue"),lty = c(2,1))
#Example of plotting of coefficients and their confidence bands
plot(summary(rq(foodexp~income,tau = 1:49/50,data=engel)))
#Example to illustrate inequality constrained fitting
n <- 100
p <- 5
X <- matrix(rnorm(n*p),n,p)
y <- .95*apply(X,1,sum)+rnorm(n)
#constrain slope coefficients to lie between zero and one
R <- cbind(0,rbind(diag(p),-diag(p)))
r <- c(rep(0,p),-rep(1,p))
rq(y~X,R=R,r=r,method="fnc")
}
\section{Method}{
The function computes an estimate on the tau-th conditional quantile
function of the response, given the covariates, as specified by the
formula argument. Like \code{lm()}, the function presumes a linear
specification for the quantile regression model, i.e. that the formula
defines a model that is linear in parameters. For non-linear quantile
regression see the package \code{nlrq()}.
The function minimizes a weighted sum of absolute
residuals that can be formulated as a linear programming problem. As
noted above, there are three different algorithms that can be chosen
depending on problem size and other characteristics. For moderate sized
problems (\eqn{n \ll 5,000, p \ll 20}{n << 5,000, p << 20}) it is recommended
that the default \code{"br"} method be used. There are several choices of methods for
computing confidence intervals and associated test statistics.
See the documentation for \code{\link{summary.rq}} for further details
and options.
}
\keyword{regression}
\references{
[1] Koenker, R. W. and Bassett, G. W. (1978). Regression quantiles,
\emph{Econometrica}, \bold{46}, 33--50.
[2] Koenker, R.W. and d'Orey (1987, 1994). Computing regression quantiles.
\emph{Applied Statistics}, \bold{36}, 383--393, and \bold{43}, 410--414.
[3] Gutenbrunner, C. Jureckova, J. (1991).
Regression quantile and regression rank score process in the
linear model and derived statistics, \emph{Annals of Statistics},
\bold{20}, 305--330.
[4] Koenker, R. W. (1994). Confidence Intervals for regression quantiles, in
P. Mandl and M. Huskova (eds.), \emph{Asymptotic Statistics}, 349--359,
Springer-Verlag, New York.
[5] Koenker, R. and S. Portnoy (1997) The Gaussian Hare and the Laplacean
Tortoise: Computability of Squared-error vs Absolute Error Estimators,
(with discussion). \emph{Statistical Science,} \bold{12}, 279-300.
[6] Koenker, R. W. (2005). \emph{Quantile Regression}, Cambridge U. Press.
There is also recent information available at the URL:
\url{http://www.econ.uiuc.edu}.
}
\seealso{
\code{\link{FAQ}},
\code{\link{summary.rq}},
\code{\link{nlrq}},
\code{\link{rqss}},
\code{\link{rq.object}},
\code{\link{rq.process.object}}
}
