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Calculate exceedance probabilities

Source: R/calc_exceedance_prob.R
calc_exceedance_prob.Rd

Calculates exceedance probabilities for a numeric vector, such as stream discharge, using a plotting-position formula. Larger values are assigned lower exceedance probabilities.

Usage

calc_exceedance_prob(
  values,
  remove_zeros = FALSE,
  alpha = 0,
  rm.zero = lifecycle::deprecated()
)

Arguments

values

Numeric vector of values, such as stream discharge.

remove_zeros

Logical. If TRUE, zero and negative values are excluded from the ranking and returned as NA. Defaults to FALSE.

alpha

Numeric plotting-position constant between 0 and 0.5. Defaults to 0, corresponding to the Weibull plotting position. Common alternatives include 0.375 (Blom), 0.4 (Cunnane), 0.44 (Gringorten), and 0.5 (Hazen).

rm.zero

[Deprecated] Use remove_zeros instead.

Value

A numeric vector of exceedance probabilities with the same length and order as values. Missing and infinite input values are returned as NA.

Details

Exceedance probabilities are calculated using the general plotting-position formula: $$P = \frac{\mathrm{rank} - \alpha}{n + 1 - 2\alpha}$$ where rank is the descending rank of each value, n is the number of ranked observations, and alpha is the plotting-position constant.

Tied values are assigned the average of the ranks they would otherwise occupy, so identical values receive identical exceedance probabilities.

References

Cunnane, C. (1978). Unbiased plotting positions — A review. Journal of Hydrology, 37(3–4), 205–222. doi:10.1016/0022-1694(78)90017-3

Examples

flow_data <- c(10, 5, 0, 15, 8, NA, 0, 20)

calc_exceedance_prob(flow_data)
#> [1] 0.3750 0.6250 0.8125 0.2500 0.5000     NA 0.8125 0.1250
calc_exceedance_prob(flow_data, remove_zeros = TRUE)
#> [1] 0.5000000 0.8333333        NA 0.3333333 0.6666667        NA        NA
#> [8] 0.1666667
calc_exceedance_prob(flow_data, alpha = 0.4)
#> [1] 0.36111111 0.63888889 0.84722222 0.22222222 0.50000000         NA 0.84722222
#> [8] 0.08333333