Difference in formula for Target wait between help file and function.

[chrismainey]

Difference between formula in help and that what the function actually does.

Help file suggests:

• Division by waiting time instead of multiplication. This is fine, as it is reversed and equivalent mathematically, just different to help.
• Function divided sum of squared coefficients of variation by 2, and formula in help does not. Not sure which is correct.

NHSRwaitinglist/R/calc_target_capacity.R

Lines 6 to 43 in f1f5434

	#' capacity = demand + (cvd**2 + cvc**2) / waiting_time
	#'
	#' where
	#' cvd = coefficient of variation of time between arrivals
	#' cvd = coefficient of variation of service times
	#' waiting_time = target_wait / factor
	#'
	#' @param demand Numeric value of rate of demand in same units as target wait
	#' e.g. if target wait is weeks, then demand in units of patients/week.
	#' @param target_wait Numeric value of number of weeks that has been set as the
	#' target within which the patient should be seen.
	#' @param factor the amount we divide the target by in the waiting list
	#' e.g. if target is 52 weeks the mean wait should be 13 for a factor of 4
	#' @param cv_demand coefficient of variation of time between arrivals
	#' @param cv_capacity coefficient of variation between removals due to
	#' operations completed
	#'
	#' @return numeric. The capacity required to achieve a target waiting time.
	#' @export
	#'
	#' @examples
	#'
	#' demand <- 4 # weeks
	#' target_wait <- 52 # weeks
	#'
	#' # number of operations per week to have mean wait of 52/4
	#' calc_target_capacity(demand, target_wait)
	#'
	calc_target_capacity <- function(
	demand,
	target_wait,
	factor = 4,
	cv_demand = 1,
	cv_capacity = 1
	) {
	check_class(demand, target_wait, factor, cv_demand, cv_capacity)
	target_cap <-
	demand + ((cv_demand**2 + cv_capacity**2) / 2) * (factor / target_wait)