File: dose.p.R

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# file MASS/R/dose.p.R
# copyright (C) 1994-9 W. N. Venables and B. D. Ripley
#
#  This program is free software; you can redistribute it and/or modify
#  it under the terms of the GNU General Public License as published by
#  the Free Software Foundation; either version 2 or 3 of the License
#  (at your option).
#
#  This program is distributed in the hope that it will be useful,
#  but WITHOUT ANY WARRANTY; without even the implied warranty of
#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#  GNU General Public License for more details.
#
#  A copy of the GNU General Public License is available at
#  http://www.r-project.org/Licenses/
#
dose.p <- function(obj, cf = 1:2, p = 0.5) {
  eta <- family(obj)$linkfun(p)
  b <- coef(obj)[cf]
  x.p <- (eta - b[1L])/b[2L]
  names(x.p) <- paste("p = ", format(p), ":", sep = "")
  pd <-  -cbind(1, x.p)/b[2L]
  SE <- sqrt(((pd %*% vcov(obj)[cf, cf]) * pd) %*% c(1, 1))
  res <- structure(x.p, SE = SE, p = p)
  class(res) <- "glm.dose"
  res
}

print.glm.dose <- function(x, ...)
{
  M <- cbind(x, attr(x, "SE"))
  dimnames(M) <- list(names(x), c("Dose", "SE"))
  x <- M
  NextMethod("print")
}