Equitable Skill Scores for Categorical Forecasts

Abstract
Many skill scores used to evaluate categorical forecasts of discrete variables are inequitable, in the sense that constant forecasts of some events lead to better scores than constant forecasts of other events. Inequitable skill scores may encourage forecasters to favor some events at the expense of other events, thereby producing forecasts that exhibit systematic biases or other undesirable characteristics. This Paper describes a method of formulating equitable skill scores for categorical forecasts of nominal and ordinal variables. Equitable skill scores are based on scoring matrices, which assign scores to the various combinations of forecast and observed events. The basic tenets of equitability require that (i) all constant forecasts—and random forecosts—receive the same expected score, and (ii) the elements of scoring matrices do not depend on the elements of performance matrices. Scoring matrices are assumed here to be symmetric and to possess other reasonable properties related to the natu... Abstract Many skill scores used to evaluate categorical forecasts of discrete variables are inequitable, in the sense that constant forecasts of some events lead to better scores than constant forecasts of other events. Inequitable skill scores may encourage forecasters to favor some events at the expense of other events, thereby producing forecasts that exhibit systematic biases or other undesirable characteristics. This Paper describes a method of formulating equitable skill scores for categorical forecasts of nominal and ordinal variables. Equitable skill scores are based on scoring matrices, which assign scores to the various combinations of forecast and observed events. The basic tenets of equitability require that (i) all constant forecasts—and random forecosts—receive the same expected score, and (ii) the elements of scoring matrices do not depend on the elements of performance matrices. Scoring matrices are assumed here to be symmetric and to possess other reasonable properties related to the natu...

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