#' Variance-Covariance Matrix for Rank-Based Regression
#'
#' Returns the variance-covariance matrix of the regression estimates from an
#' object of type rfit.
#'
#'
#' @param object an object of type rfit
#' @param intercept logical. If TRUE include the variance-covariance estimates
#' corresponding to the intercept
#' @param \dots additional arguments
#' @author John Kloke
#' @seealso \code{\link{rfit}}
#' @references Hettmansperger, T.P. and McKean J.W. (2011), \emph{Robust
#' Nonparametric Statistical Methods, 2nd ed.}, New York: Chapman-Hall.
#' @export vcov.rfit
vcov.rfit <- function (object, intercept = NULL, ...) {
Q<-qr.Q(object$qrx1)
q1<-Q[,1]
Q2<-Q[,2:object$qrx1$rank]
xxpxi<-object$x%*%chol2inv(chol(crossprod(object$x)))
A1<-crossprod(q1,xxpxi) ; A2<-crossprod(Q2,xxpxi)
object$taushat^2*crossprod(A1)+object$tauhat^2*crossprod(A2)
}
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