R/var_of_percentage.R

In kim: A Toolkit for Behavioral Scientists

#' Variance of a percentage
#'
#' Calculate the variance of a percentage.
#' See Fowler, Jr. (2014, p. 34, ISBN: 978-1-4833-1240-8)
#'
#' @param percent a vector of percentages; each of the percentage values
#' must be between 0 and 100
#' @param n a vector of sample sizes; number of observations used
#' to calculate each of the percentage values
#' @examples
#' var_of_percentage(percent = 40, n = 50)
#' var_of_percentage(percent = 50, n = 10)
#' @export
var_of_percentage <- function(percent = NULL, n = NULL) {
  # check that the percentage is in the expected range
  if (any(percent < 0)) {
    stop(paste0(
      "Please check your input.",
      "\nA percentage value for this function cannot be less than 0."))
  }
  if (any(percent > 100)) {
    stop(paste0(
      "Please check your input.",
      "\nA percentage value for this function cannot be greater than 100."))
  }
  # check the n
  if (is.null(n)) {
    stop("Please provide an input for the argument `n`.")
  }
  # convert percentage to a proportion
  p <- percent / 100
  return((p * (1 - p) / n) * 100)
}