#' Find standard error for survival quantile
#'
#' @param timevar1 Vector of observed survival times for sample 1 (control).
#' @param censor1 Vector of censoring indicators for sample 1 (1 = uncensored, 0 = censored).
#' @param timevar2 Vector of observed survival times for sample 2 (treatment).
#' @param censor2 Vector of censoring indicators for sample 2 (1 = uncensored, 0 = censored).
#' @param q Quantile of interest (in terms of CDF). Default is median.
#' @param B Number of bootstrap samples.
#' @param seed Seed number (for reproducibility).
#' @param plots Logical. TRUE to show plot of cumulative distribution functions.
#' @return Returns quantile estimate, bootstrapped standard error, test statistic, and two-sided p-value.
#' @examples
#' #Reference: Survival Analysis Techniques for Censored and Truncated Data.
#' #Klein and Moeschberger (1997) Springer.
#' #Data: Chapter 7.6 Example 7.9 (p. 211)
#' library(controlTest)
#' t1 <- c(1, 63, 105, 129, 182, 216, 250, 262, 301, 301,
#' 342, 354, 356, 358, 380, 383, 383, 338, 394, 408, 460, 489,
#' 499, 523, 524, 535, 562, 569, 675, 676, 748, 778, 786, 797,
#' 955, 968, 1000, 1245, 1271, 1420, 1551, 1694, 2363, 2754, 2950)
#' t2 <- c(17, 42, 44, 48, 60, 72, 74, 95, 103, 108, 122, 144, 167, 170,
#' 183, 185, 193, 195, 197, 208, 234, 235, 254, 307, 315, 401, 445,
#' 464, 484, 528, 542, 547, 577, 580, 795, 855, 1366, 1577, 2060,
#' 2412, 2486, 2796, 2802, 2934, 2988)
#' c1 <- c(rep(1, 43), 0, 0)
#' c2 <- c(rep(1, 39), rep(0, 6))
#' quantileControlTest(t1, c1, t2, c2, q = 0.5, B = 500)
#'
#'@details It is important to note the possiblilty that the estimated quantile may not be estimable in our bootstrap samples. In such cases
#' the largest observed survival time will be considered as an estimate for the quantile.
#'
#' @references
#' Li, G., Tiwari, R.C., and Wells, M. (1996). "Quantile Comparison Functions in Two-Sample Problems: With Applications to Comparisons of Diagnostic Markers." Journal of the American Statistical Association, 91, 689-698.
#'
#' Chakraborti, S., and Mukerjee, R. (1989), "A Confidence Interval for a Measure Associated With the Comparison of a Treatment With a Control," South African Statistical Journal, 23, 219-230.
#'
#' Gastwirth, J. L., and Wang, J. L. (1988), "Control Percentile Test for Censored Data," Journal of Statistical Planning and Inference, 18, 267-276.
#' @export
#' @import survival
#' @importFrom graphics legend lines plot
#' @importFrom stats pnorm qnorm quantile sd stepfun
#'
quantileControlTest <- function(timevar1, censor1, timevar2, censor2, q = 0.5, B = 1000, seed = 1234, plots = FALSE) {
#- Checking for silly errors
if (q < 0 | q > 1 ) {
stop("q should be between 0 and 1.")
}
if (B <= 0) {
stop("B should be a positive integer")
}
if (seed <= 0) {
stop("Seed should be a positive integer")
}
if (length(timevar1) != length(censor1)) {
stop("Length of timevar1 and censor1 should be the same")
}
if (length(timevar2) != length(censor2)) {
stop("Length of timevar2 and censor2 should be the same")
}
set.seed(seed)
fit1 <- survfit(Surv(timevar1, censor1) ~ 1, conf.type = "none")
fit2 <- survfit(Surv(timevar2, censor2) ~ 1, conf.type = "none")
F1.inv <- unname(quantile(fit1, prob = q)) #Quantile in terms of CDF
F2.inv <- unname(quantile(fit2, prob = q)) #Quantile in terms of CDF
# If quantiles do not exist for both samples
if(is.na(F1.inv)) {
stop(paste0("Estimate of ", round(q*100, 2), "-th quantile for sample 1 (control) not found. Program Stopped."))
} else if(is.na(F2.inv)) {
stop(paste0("Estimate of ", round(q*100, 2), "-th quantile for sample 2 (treatment) not found. Program Stopped."))
}
# Calculate F2(F1.inv(p))
Qp <- function(t1, c1, t2, c2) {
fit1 <- survfit(Surv(t1, c1) ~ 1, conf.type = "none")
fit2 <- survfit(Surv(t2, c2) ~ 1, conf.type = "none")
F1.inv <- unname(quantile(fit1, prob = q)) #F1.inv(p)
if (is.na(F1.inv)) {
warning(paste0("Estimate of ", round(q*100, 2), "-th quantile could not be calculated for bootstrap sample. Largest observed survival time was used instead."))
F1.inv <- max(t1)
}
F2 <- stepfun(fit2$time, c(0, 1 - fit2$surv)) #CDF of F2
out <- F2(F1.inv) #F2(F1.inv(p))
return(out)
}
Q <- Qp(timevar1, censor1, timevar2, censor2)
# Bootstrap
b.est <- numeric(B)
for (i in 1:B) {
boot1 <- sample(1:length(timevar1), replace = TRUE)
t1.boot <- timevar1[boot1]
c1.boot <- censor1[boot1]
boot2 <- sample(1:length(timevar2), replace = TRUE)
t2.boot <- timevar2[boot2]
c2.boot <- censor2[boot2]
b.est[i] <- Qp(t1.boot, c1.boot, t2.boot, c2.boot)
}
se <- sd(b.est)
Z <- (Q - (1 - q)) / se
pval <- 2 * (1 - pnorm(abs(Z)))
#- Plots
if (plots == TRUE) {
plot( (1 - fit1$surv) ~ fit1$time, col = "red", type = "s", ylab = "F(x)",
xlab = "Time", main = "Estimated CDF for Control and Trt. Group")
lines((1 - fit2$surv) ~ fit2$time, type = "s", lty = 2, col = "blue")
legend("bottomright", c("CDF Estimate for Control Group", "CDF Estimate for Trt. Group"),
lty = c(1, 2), col = c("red", "blue"), bty = "n", cex = 0.8)
}
out <- list()
out$cdf_quantile <- round(q, 2)
out$sample1 <- F1.inv
out$sample2 <- F2.inv
out$Z <- Z
out$se <- se
out$pval <- round(pval, 3)
out$B <- B
return(out)
}
Add the following code to your website.
For more information on customizing the embed code, read Embedding Snippets.