Common Sources of Error with tbl_survfit() — tbl_survfit

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.

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
```
survfit(Surv(ttdeath, death) ~ trt, trial) %>%
  tbl_survfit(times = c(12, 24))
```
Use this code
```
trial %>%
  select(ttdeath, death, trt) %>%
  tbl_survfit(y = Surv(ttdeath, death), times = c(12, 24))
```
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
```
formula_arg <- Surv(ttdeath, death) ~ 1
data_arg <- trial
rlang::expr(survfit(!!formula_arg, !!data_arg)) %>%
  eval() %>%
  tbl_survfit(times = c(12, 24))
```