R/ci_Z.R

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

#' @title Confidence interval for mean (known sigma)
#' @description Calculates the confidence interval for the mean with known standard deviation, using the Z-statistic (normal distribution).
#' @param x Sample mean
#' @param sig Population 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 mean, and lower and upper ends of the confidence interval
#' @examples
#' x <- 80
#' sig <- 15
#' n <- 20
#' ci_z(x, sig, n)
#'
ci_z <- function (x, sig, n, conf.level = .95, digits = 2){
  alpha <- 1 - conf.level
  stderr <- sig/sqrt(n)
  upper <- x + stats::qnorm(1-alpha/2, 0, 1) * stderr
  lower <- x + stats::qnorm(alpha/2, 0, 1) * stderr
  DF <- tibble::tibble(mean = x,
                       lower = round(lower, digits),
                       upper = round(upper, digits))
  DF
}