R/dist_weibull.R

In distributional: Vectorised Probability Distributions

#' The Weibull distribution
#'
#' @description
#' `r lifecycle::badge('stable')`
#'
#' Generalization of the gamma distribution. Often used in survival and
#' time-to-event analyses.
#'
#' @inheritParams stats::dweibull
#'
#' @details
#'
#'   We recommend reading this documentation on
#'   <https://pkg.mitchelloharawild.com/distributional/>, where the math
#'   will render nicely.
#'
#'   In the following, let \eqn{X} be a Weibull random variable with
#'   success probability `p` = \eqn{p}.
#'
#'   **Support**: \eqn{R^+} and zero.
#'
#'   **Mean**: \eqn{\lambda \Gamma(1+1/k)}, where \eqn{\Gamma} is
#'   the gamma function.
#'
#'   **Variance**: \eqn{\lambda [ \Gamma (1 + \frac{2}{k} ) - (\Gamma(1+ \frac{1}{k}))^2 ]}
#'
#'   **Probability density function (p.d.f)**:
#'
#'   \deqn{
#'     f(x) = \frac{k}{\lambda}(\frac{x}{\lambda})^{k-1}e^{-(x/\lambda)^k}, x \ge 0
#'   }
#'
#'   **Cumulative distribution function (c.d.f)**:
#'
#'   \deqn{F(x) = 1 - e^{-(x/\lambda)^k}, x \ge 0}
#'
#'   **Moment generating function (m.g.f)**:
#'
#'   \deqn{\sum_{n=0}^\infty \frac{t^n\lambda^n}{n!} \Gamma(1+n/k), k \ge 1}
#'
#' @seealso [stats::Weibull]
#'
#' @examples
#' dist <- dist_weibull(shape = c(0.5, 1, 1.5, 5), scale = rep(1, 4))
#'
#' dist
#' mean(dist)
#' variance(dist)
#' skewness(dist)
#' kurtosis(dist)
#'
#' generate(dist, 10)
#'
#' density(dist, 2)
#' density(dist, 2, log = TRUE)
#'
#' cdf(dist, 4)
#'
#' quantile(dist, 0.7)
#'
#' @name dist_weibull
#' @export
dist_weibull <- function(shape, scale){
  shape <- vec_cast(shape, double())
  scale <- vec_cast(scale, double())
  if(any(shape[!is.na(shape)] < 0)){
    abort("The shape parameter of a Weibull distribution must be non-negative.")
  }
  if(any(scale[!is.na(scale)] <= 0)){
    abort("The scale parameter of a Weibull distribution must be strictly positive.")
  }
  new_dist(shape = shape, scale = scale, class = "dist_weibull")
}

#' @export
format.dist_weibull <- function(x, digits = 2, ...){
  sprintf(
    "Weibull(%s, %s)",
    format(x[["shape"]], digits = digits, ...),
    format(x[["scale"]], digits = digits, ...)
  )
}

#' @export
density.dist_weibull <- function(x, at, ...){
  stats::dweibull(at, x[["shape"]], x[["scale"]])
}

#' @export
log_density.dist_weibull <- function(x, at, ...){
  stats::dweibull(at, x[["shape"]], x[["scale"]], log = TRUE)
}

#' @export
quantile.dist_weibull <- function(x, p, ...){
  stats::qweibull(p, x[["shape"]], x[["scale"]])
}

#' @export
cdf.dist_weibull <- function(x, q, ...){
  stats::pweibull(q, x[["shape"]], x[["scale"]])
}

#' @export
generate.dist_weibull <- function(x, times, ...){
  stats::rweibull(times, x[["shape"]], x[["scale"]])
}

#' @export
mean.dist_weibull <- function(x, ...){
  x[["scale"]] * gamma(1 + 1/x[["shape"]])
}

#' @export
covariance.dist_weibull <- function(x, ...){
  x[["scale"]]^2 * (gamma(1 + 2/x[["shape"]]) - gamma(1 + 1/x[["shape"]])^2)
}

#' @export
skewness.dist_weibull <- function(x, ...) {
  mu <- mean(x)
  sigma <- sqrt(variance(x))
  r <- mu / sigma
  gamma(1 + 3/x[["shape"]]) * (x[["scale"]]/sigma)^3 - 3*r - 3^r
}

#' @export
kurtosis.dist_weibull <- function(x, ...) {
  mu <- mean(x)
  sigma <- sqrt(variance(x))
  gamma <- skewness(x)
  r <- mu / sigma
  (x[["scale"]]/sigma)^4 * gamma(1 + 4/x[["shape"]]) - 4*gamma*r -6*r^2 - r^4 - 3
}