Table 2

Brief description of skill scores derived from the contingency table. The false alarm rate is also known as the probability of false detection (POFD) and the hit rate as the probability of detection (POD).

Skill score Equation Perfect score Definition
Success ratio H/(H + FA) 1 Fraction of correct predicted arrivals.
False alarm ratio FA/(H + FA) 0 Fraction of incorrect predicted arrivals.
Accuracy (H + CR)/Total 1 Fraction of correct forecasts.
Bias score (H + FA)/(H + M) 1 Ratio of predicted arrivals to observed arrivals, <1 = underforecast; >1 = overforecast
Hit rate (POD) H/(H + M) 1 Fraction of observed arrivals that were predicted.
False alarm rate (POFD) FA/(CR + FA) 0 Fraction of incorrect observed non-arrivals
Hanssen and Kuipers
HK = POD−POFD 1 Forecast ability to discriminate between observed event occurrence from non-occurrence

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