#' Common Sources of Error with `tbl_survfit()`
#'
#' @description When functions `add_n()` and `add_p()` are run after `tbl_survfit()`,
#' the original call to `survival::survfit()`
#' is extracted and the `formula=` and `data=` arguments are used to calculate
#' the N or p-value.
#'
#' When the values of the `formula=` and `data=` are unavailable, the functions
#' cannot execute. Below are some tips to modify your code to ensure all functions
#' run without issue.
#'
#' 1. Let `tbl_survfit()` construct the `survival::survfit()` for you by passing
#' a data frame to `tbl_survfit()`. The survfit model will be constructed
#' in a manner ensuring the formula and data are available.
#' This only works if you have a stratified model.
#'
#' Instead of the following line
#'
#' ```r
#' survfit(Surv(ttdeath, death) ~ trt, trial) %>%
#' tbl_survfit(times = c(12, 24))
#' ```
#' Use this code
#'
#' ```r
#' trial %>%
#' select(ttdeath, death, trt) %>%
#' tbl_survfit(y = Surv(ttdeath, death), times = c(12, 24))
#' ```
#'
#' 2. Construct an expression of the `survival::survfit()` before evaluating it.
#' Ensure the formula and data are available in the call by using the tidyverse
#' bang-bang operator, `!!`.
#'
#' Use this code
#'
#' ```r
#' formula_arg <- Surv(ttdeath, death) ~ 1
#' data_arg <- trial
#' rlang::expr(survfit(!!formula_arg, !!data_arg)) %>%
#' eval() %>%
#' tbl_survfit(times = c(12, 24))
#' ```
#' @keywords internal
#' @name tbl_survfit_errors
NULL
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