NCL Home > Documentation > Functions > Statistics, Extreme values

extval_return_period

Calculates the period of an event (eg, flood, heat wave, drought) occurring given an average event recurrence interval and specified probability level.

Available in version 6.4.0 and later.

Prototype

```	function extval_return_period (
Tr [*] : numeric,
Pr [*] : numeric
)

return_val [ variable of type double if Tr .or. Nr is "double"; else, type "float" is returned]
```

Arguments

Tr

A scalar or one dimensional numeric array containing the average event recurrence interval(s).

Pr

A scalar or one dimensional numeric array specifying the probability [0 <= Pr <= 1).

Return value

A variable of type double if Tr .or. Pr is "double"; else, type "float" is returned.

Description

Estimate the return period (recurrence interval):

```          Nr = log(Pr)/log(1-(1.0/Tr))
```

Examples

Example 1: If an 'event' occures every 100 years (=Tr), for how many years should the specified level of confidence (=Pr) apply?

```  Ta = 100                          ; recurrence years (days ... whatever)
Pr = 0.95                         ; user specified
Nr = extval_return_period(Ta, Pr) ; Nr=5.1 years at 0.95 probability
```

Example 2: Illustrate returned dimensionality and numeric type.

```  Ta  = 100      ;
Pa  =  0.95
Na  = extval_return_period(Ta, Pa)    ; Nr is scalar; float

Tb  = (/ 5.4, 23.7, 50, 100 /)
Pb  = 0.99
Nb  = extval_return_period(Tb, Pb)    ; Nb(4); float

Tc  = 100d0
Pc  = (/ 0.90, 0.95, 0.975, 0.99, 0.999/)
Nc  = extval_return_period(Tc, Pc)    ; Nc(5); double

Td  = (/ 20, 50, 100, 100 /)
Pd  = (/ 0.90, 0.95, 0.975, 0.99, 0.999 /)
Nd  = extval_return_period(Td, Pd)    ; Nd(4,5); float
```