Forecast skill as a function of lead-time for Proton Event probability forecasts during the period 1986 to 2006. The skill of forecasts produced by SEC is compared to that of forecasts produced by short-term (30-day) climatology and 1-day persistence. This skill metric is based on the relative error of the forecasts with respect to constant forecasts of sample climatology (the mean observation during the period) and is sometimes called the “prediction efficiency.” The upper bound for this metric is "one" and there is no lower bound. Negative values indicate no skill above constant forecasts of sample climatology.

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