local({
  previous_options <- options(warn = 2, digits = 8, stringsAsFactors = FALSE)
  on.exit(options(previous_options), add = TRUE)
  visits <- c(8, 10, 12)
  population_variance <- mean((visits - mean(visits))^2)
  cat("sample variance:", var(visits), "\n")
  cat("sample sd:", sd(visits), "\n")
  cat("population calculation:", sprintf("%.3f", population_variance), "\n")
  invisible(NULL)
})