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#' @title Survival Fragility Function
#' @description Compute the fragility of a coefficient in a survival test, i.e. the number of removed observations it would take to make a significant-result non-significant. Uses the coxph() function from the survival package.
#'
#' @param formula Model formula which will be evaluated by coxph()
#' @param data Dataframe with values for model forma, passed to coxph()
#' @param covariate Vector of covariates to find fragility index for. Default is all covariates in formula
#' @param conf.level Significance level
#' @param verbose Logical indicating if function will return verbose results or only fragility index
#'
#' @importFrom survival coxph
#' @importFrom survival Surv
#' @importFrom stats glm
#' @importFrom stats terms
#' @importFrom stats terms.formula
#' @importFrom stats anova
#' @importFrom stats update
#' @importFrom stats as.formula
#' @importFrom stats residuals
#' @importFrom stats anova
#' @examples
#' library(survival); data <- lung
#' data$status = lung$status - 1 # recode status as a 0/1 variable
#'
#' survivalfragility(Surv(time, status) ~ pat.karno + strata(inst),
#' data, covariate = "pat.karno")
#'
#' survivalfragility(Surv(time, status) ~ pat.karno + ph.karno + strata(inst),
#' data, verbose = TRUE)
#' #algorithm does not converge for strata(inst)
#'
#' survivalfragility(Surv(time, status) ~ pat.karno + ph.karno + strata(inst),
#' data, covariate = c("pat.karno","ph.karno"))
#'
#'
#' @return If verbose is FALSE, returns a list with fragility indices for selected covariates. If
#' verbose is TRUE, returns a list with p-values for each fragility index
#' at each iteration of the algorithm.
#'
#' @export survivalfragility
survivalfragility <- function(formula, data, covariate = "all.factors.default", conf.level = 0.95, verbose = FALSE){
if ("all.factors.default" %in% covariate) {
object <- terms.formula(formula)
covariate.names <- attr(object, "term.labels")
} else {
covariate.names <- covariate
}
result.store <- vector("list", length(covariate.names))
names(result.store) = covariate.names
for (i in 1:length(covariate.names)) {
if (verbose == TRUE) {
result <- survivalfragilityinternal(formula, data, covariate.names[i], conf.level)
} else{
result <- survivalfragilityinternal(formula, data, covariate.names[i], conf.level)
result <- result[1]
}
result.store[[paste(covariate.names[i])]] <- result
}
return(result.store)
}
survivalfragilityinternal <- function(formula, data, covariate, conf.level) {
alpha <- (1 - conf.level)
model <- coxph(formula, data)
na.data <- model$na.action
indata <- data[-na.data, ]
model <- coxph(formula, indata)
formula <- model$formula
nullmodel <- update(model, as.formula(paste(".~.-", covariate)))
delta.resid <- residuals(model, type="deviance") - residuals(nullmodel, type="deviance") #which residuals to use?
index <- c(1:length(delta.resid))
surv.status = formula[2][[1]][[3]]
ordering <- cbind(index, delta.resid, indata[ ,paste(surv.status)])
ordering <- cbind(ordering, (ordering[ ,2] - ordering[ ,3] * 2 * ordering[ ,2]))
ordering <- ordering[order(-ordering[ ,4]), ]
pval <- anova(model, nullmodel)$'P(>|Chi|)'[2]
index <- 1
iter <- 0
pvalues <- pval
not.significant <- FALSE
if (pval > alpha) {
not.significant <- TRUE
}
while (pval <= alpha & (iter <= nrow(indata) - 3 * length(terms(formula)))) {
points <- ordering[1:index,1]
modified.data <- indata[-points, ]
newmodel <- coxph(formula, modified.data)
newnullmodel <- update(newmodel, as.formula(paste(".~.-", covariate)))
pval <- anova(newmodel, newnullmodel)$'P(>|Chi|)'[2]
if (is.na(pval)) {
return(list(fragility.index = NA, point.diagnostics = "algorithm did not converge"))
}
index <- index + 1
iter <- iter + 1
pvalues <- append(pvalues, pval)
}
if (iter >= nrow(indata) - 3 * length(terms(formula))) {
return(list(fragility.index = NA, point.diagnostics = "algorithm did not converge"))
}
resulting.pval <- pvalues[-1]
if (not.significant) {
resulting.pval <- anova(model, nullmodel)$'P(>|Chi|)'[2]
}
fragility.index <- index - 1
point.diagnostics <- indata[1:(index-1), ]
if (not.significant) {
point.diagnostics <- paste("No points removed. Covariate already not significant at confidence level", conf.level)
}
point.diagnostics <- cbind(point.diagnostics, resulting.pval)
return(list(fragility.index = fragility.index, point.diagnostics = point.diagnostics))
}
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