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Gerrity Score

Note: Though software routines are provided for the calculation of the Gerrity Skill Score, results are not as yet displayed on the website.

The Gerrity Skill Score (GSS) (Gerrity, 1992) is the recommended skill score for the three by three contingency tables calculated for Level 3 of the SVSLRF. It is detailed on page 16 of the manual (attachments II.8), of which some is repeated here.

The definition of the GSS uses a scoring matrix sij (i = 1,...,3), which is a tabulation of the reward or penalty every forecast/observation outcome (represented by the contingency table) will be accorded:

Where the scoring matrix is given by:


Note that GSS is computed using the sample probabilities, not those on which the original categorisations were based (i.e. 0.33, 0.33, 0.33).



Gerrity, J.P., 1992, A Note on Gandin and Murphy's Equitable Skill Score., Monthly Weather Review, 120, 2709-2712.

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