Small helper function for suggesting number of lags in Ljung-Box test per Rob Hyndman advice in fpp3

suggest_ljung_box_lags.R

# From: https://otexts.com/fpp3/diagnostics.html#portmanteau-tests-for-autocorrelation
#
# We suggest using l = 10 for non-seasonal data and l = 2 * m for seasonal data, where m is the period of
# seasonality. However, the test is not good when l is large, so
# if these values are larger than T / 5 then use l = T / 5.

# Translating into R function:
#  * n_obs - number of observations in the series (referred to as T in the text)
#  * n_seasonal_periods - NA if non-seasonal; 4 if quarterly; 12 if monthly; etc.

suggest_ljung_box_lags <- function(n_obs, n_seasonal_periods = 4) {
  if (is.na(n_seasonal_periods)) {
    # We suggest using l = 10 for non-seasonal data
    suggested_n_lags <- 10
  } else {
    # We suggest using ... l = 2*m for seasonal data,
    # where m is the period of seasonality
    suggested_n_lags <- 2 * n_seasonal_periods
  }

  # However, the test is not good when l is large, so
  # if these values are larger than T / 5 then use l = T / 5
  max_lags <- floor(n_obs / 5)
  if (suggested_n_lags > max_lags) {
    suggested_n_lags <- max_lags
  }

  return(suggested_n_lags)
}