R/binomCI.R

In stamats/MKmisc: Miscellaneous Functions from M. Kohl

## Confidence Intervals for Binomial Proportions
binomCI <- function(x, n, conf.level = 0.95, method = "wilson", rand = 123){
    if (!is.na(pmatch(method, "wilson")))
        method <- "wilson"
    METHODS <- c("wald", "wilson", "agresti-coull", "jeffreys", "modified wilson",
                 "modified jeffreys", "clopper-pearson", "arcsine", "logit", "witting")
    method <- pmatch(method, METHODS)

    if (is.na(method))
        stop("invalid method")
    if (method == -1)
        stop("ambiguous method")

    if(length(x) != 1)
        stop("'x' has to be of length 1 (number of successes)")
    if(length(n) != 1)
        stop("'n' has to be of length 1 (number of trials)")
    if(length(conf.level) != 1)
        stop("'conf.level' has to be of length 1 (confidence level)")
    if(conf.level < 0.5 | conf.level > 1)
        stop("'conf.level' has to be in [0.5, 1]")

    x <- as.integer(x)
    n <- as.integer(n)
    alpha <- 1 - conf.level
    kappa <- qnorm(1-alpha/2)
    p.hat <- x/n
    q.hat <- 1 - p.hat
    Infos <- NULL

    if(method == 1){ # wald
        est <- p.hat
        term2 <- kappa*sqrt(p.hat*q.hat)/sqrt(n)
        CI.lower <- max(0, p.hat - term2)
        CI.upper <- min(1, p.hat + term2)
        Infos <- term2/kappa
        names(Infos) <- "standard error of prob"
    }
    if(method == 2){ # wilson
        est <- p.hat
        term1 <- (x + kappa^2/2)/(n + kappa^2)
        term2 <- kappa*sqrt(n)/(n + kappa^2)*sqrt(p.hat*q.hat + kappa^2/(4*n))
        CI.lower <-  max(0, term1 - term2)
        CI.upper <- min(1, term1 + term2)
        Infos <- term2/kappa
        names(Infos) <- "standard error of prob"
    }
    if(method == 3){ # agresti-coull
        x.tilde <- x + kappa^2/2
        n.tilde <- n + kappa^2
        p.tilde <- x.tilde/n.tilde
        q.tilde <- 1 - p.tilde
        est <- p.tilde
        term2 <- kappa*sqrt(p.tilde*q.tilde)/sqrt(n.tilde)
        CI.lower <- max(0, p.tilde - term2)
        CI.upper <- min(1, p.tilde + term2)
        Infos <- term2/kappa
        names(Infos) <- "standard error of prob"
    }
    if(method == 4){ # jeffreys
        est <- p.hat
        if(x == 0)
            CI.lower <- 0
        else
            CI.lower <- qbeta(alpha/2, x+0.5, n-x+0.5)
        if(x == n)
            CI.upper <- 1
        else
            CI.upper <- qbeta(1-alpha/2, x+0.5, n-x+0.5)
    }
    if(method == 5){ # modified wilson
        est <- p.hat
        term1 <- (x + kappa^2/2)/(n + kappa^2)
        term2 <- kappa*sqrt(n)/(n + kappa^2)*sqrt(p.hat*q.hat + kappa^2/(4*n))
        if((n <= 50 & x %in% c(1, 2)) | (n >= 51 & x %in% c(1:3)))
            CI.lower <- 0.5*qchisq(alpha, 2*x)/n
        else
            CI.lower <-  max(0, term1 - term2)

        if((n <= 50 & x %in% c(n-1, n-2)) | (n >= 51 & x %in% c(n-(1:3))))
            CI.upper <- 1 - 0.5*qchisq(alpha, 2*(n-x))/n
        else
            CI.upper <- min(1, term1 + term2)
        Infos <- term2/kappa
        names(Infos) <- "standard error of prob"
    }
    if(method == 6){ # modified jeffreys
        est <- p.hat
        if(x == n)
            CI.lower <- (alpha/2)^(1/n)
        else{
            if(x <= 1)
                CI.lower <- 0
            else
                CI.lower <- qbeta(alpha/2, x+0.5, n-x+0.5)
        }
        if(x == 0)
            CI.upper <- 1 - (alpha/2)^(1/n)
        else{
            if(x >= n-1)
                CI.upper <- 1
            else
                CI.upper <- qbeta(1-alpha/2, x+0.5, n-x+0.5)
        }
    }
    if(method == 7){ # clopper-pearson
        est <- p.hat
        CI.lower <- qbeta(alpha/2, x, n-x+1)
        CI.upper <- qbeta(1-alpha/2, x+1, n-x)
    }
    if(method == 8){ # arcsine
        p.tilde <- (x + 0.375)/(n + 0.75)
        est <- p.tilde
        CI.lower <- sin(asin(sqrt(p.tilde)) - 0.5*kappa/sqrt(n))^2
        CI.upper <- sin(asin(sqrt(p.tilde)) + 0.5*kappa/sqrt(n))^2
    }
    if(method == 9){ # logit
        est <- p.hat
        lambda.hat <- log(x/(n-x))
        V.hat <- n/(x*(n-x))
        lambda.lower <- lambda.hat - kappa*sqrt(V.hat)
        lambda.upper <- lambda.hat + kappa*sqrt(V.hat)
        CI.lower <- exp(lambda.lower)/(1 + exp(lambda.lower))
        CI.upper <- exp(lambda.upper)/(1 + exp(lambda.upper))
    }
    if(method == 10){ # witting
        set.seed(rand)
        x.tilde <- x + runif(1, min = 0, max = 1)
        pbinom.abscont <- function(q, size, prob){
            v <- trunc(q)
            term1 <- pbinom(v-1, size = size, prob = prob)
            term2 <- (q - v)*dbinom(v, size = size, prob = prob)
            return(term1 + term2)
        }
        qbinom.abscont <- function(p, size, x){
            fun <- function(prob, size, x, p){
                pbinom.abscont(x, size, prob) - p
            }
            uniroot(fun, interval = c(0, 1), size = size, x = x, p = p)$root
        }
        est <- p.hat
        CI.lower <- qbinom.abscont(1-alpha, size = n, x = x.tilde)
        CI.upper <- qbinom.abscont(alpha, size = n, x = x.tilde)
    }

    CI <- matrix(c(CI.lower, CI.upper), nrow = 1)
    rownames(CI) <- "prob"
    colnames(CI) <- c(paste(alpha/2*100, "%"), paste((1-alpha/2)*100, "%"))
    attr(CI, "conf.level") <- conf.level
    names(est) <- "prob"

    return(structure(list("estimate" = est, "conf.int" = CI, "Infos" = Infos,
                          "method" = paste(METHODS[method], "confidence interval")),
                     class = "confint"))
}