R/ci_chisq.R

In maxgav13/GMisc: Miscellaneous Functions for Geology (and its areas)

#' @title Confidence interval for variance (standard deviation) of a sample
#' @description Calculates the confidence interval for the variance (standard deviation) of a sample (ChiSquare distribution).
#' @param s Sample standard deviation
#' @param n Sample size
#' @param conf.level Confidence level to use for the confidence interval (Default is 0.95)
#' @param digits Number of digits to round to (Default is 2)
#' @export
#' @return A tibble with the standard deviation and variance, degrees of freedom, and lower and upper ends of the confidence interval
#' @examples
#' s <- 0.535
#' n <- 10
#' ci_chisq(s, n)
#'
ci_chisq <- function (s, n, conf.level = .95, digits = 2){
  alpha <- 1 - conf.level
  upper <- ((n-1) * s^2)/stats::qchisq(alpha/2, n-1)
  lower <- ((n-1) * s^2)/stats::qchisq(1-alpha/2, n-1)
  DF <- tibble::tibble(stat = c("sd", "var"),
                       value = round(c(s, s^2),
                                     digits) ,
                       df = n-1,
                       lower = c(round(sqrt(lower), digits),
                                 round(lower, digits)),
                       upper = c(round(sqrt(upper), digits),
                                 round(upper, digits)))
  DF
}