1\name{sm.sigma2.compare} 2 3\alias{sm.sigma2.compare} 4 5\title{Comparison across two groups of the error standard deviation in 6 nonparametric regression with two covariates.} 7 8\description{This function compares across two groups, in a hypothesis 9 test, the error standard deviation in nonparametric regression 10 with two covariates.} 11 12\usage{sm.sigma2.compare(x1, y1, x2, y2)} 13 14\arguments{ 15 \item{x1}{a two-column matrix of covariate values for group 1.} 16 \item{y1}{a vector of responses for group 1.} 17 \item{x2}{a two-column matrix of covariate values for group 2.} 18 \item{y2}{a vector of responses for group 2.} 19 } 20 21\value{a p-value for the test of equality of standard deviations.} 22 23\section{Side Effects}{none.} 24 25\details{see the reference below.} 26 27\references{Bock, M., Bowman, A.W.\ \& Ismail, B. (2007). 28 Estimation and inference for error variance in bivariate 29 nonparametric regression. 30 \emph{Statistics \& Computing}, to appear.} 31 32\seealso{\code{\link{sm.sigma}}} 33 34\examples{ 35\dontrun{ 36with(airquality, { 37 x <- cbind(Wind, Temp) 38 y <- Ozone^(1/3) 39 group <- (Solar.R < 200) 40 sig1 <- sm.sigma(x[ group, ], y[ group], ci = TRUE) 41 sig2 <- sm.sigma(x[!group, ], y[!group], ci = TRUE) 42 print(c(sig1$estimate, sig1$ci)) 43 print(c(sig2$estimate, sig2$ci)) 44 print(sm.sigma(x[ group, ], y[ group], model = "constant", h = c(3, 5))$p) 45 print(sm.sigma(x[!group, ], y[!group], model = "constant", h = c(3, 5))$p) 46 print(sm.sigma2.compare(x[group, ], y[group], x[!group, ], y[!group])) 47}) 48}} 49\keyword{nonparametric} 50\keyword{smooth} 51