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\name{sm.sigma2.compare}
\alias{sm.sigma2.compare}
\title{Comparison across two groups of the error standard deviation in
nonparametric regression with two covariates.}
\description{This function compares across two groups, in a hypothesis
test, the error standard deviation in nonparametric regression
with two covariates.}
\usage{sm.sigma2.compare(x1, y1, x2, y2)}
\arguments{
\item{x1}{a two-column matrix of covariate values for group 1.}
\item{y1}{a vector of responses for group 1.}
\item{x2}{a two-column matrix of covariate values for group 2.}
\item{y2}{a vector of responses for group 2.}
}
\value{a p-value for the test of equality of standard deviations.}
\section{Side Effects}{none.}
\details{see the reference below.}
\references{Bock, M., Bowman, A.W.\ \& Ismail, B. (2007).
Estimation and inference for error variance in bivariate
nonparametric regression.
\emph{Statistics \& Computing}, to appear.}
\seealso{\code{\link{sm.sigma}}}
\examples{
\dontrun{
with(airquality, {
x <- cbind(Wind, Temp)
y <- Ozone^(1/3)
group <- (Solar.R < 200)
sig1 <- sm.sigma(x[ group, ], y[ group], ci = TRUE)
sig2 <- sm.sigma(x[!group, ], y[!group], ci = TRUE)
print(c(sig1$estimate, sig1$ci))
print(c(sig2$estimate, sig2$ci))
print(sm.sigma(x[ group, ], y[ group], model = "constant", h = c(3, 5))$p)
print(sm.sigma(x[!group, ], y[!group], model = "constant", h = c(3, 5))$p)
print(sm.sigma2.compare(x[group, ], y[group], x[!group, ], y[!group]))
})
}}
\keyword{nonparametric}
\keyword{smooth}
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