This is nowadays defined as the mean of the 16th and the 17th highest value. Previously, the 17th highest values was used (FOCUS 2014, p. 18). NaN values need to be set to zero in order to reproduce the values obtained by PELMO.
focus_80th(c_period, old = FALSE)
A numeric vector of values to calculate the percentile from
Should the old calculation method be used (the 17th highest value)?