Forecast skill as a function of lead-time for forecasts of USAF Estimated Ap index during the period 1989 to 2006. The skill of forecasts issued by SEC is compared to that of forecasts produced by short-term (30-day) climatology, 1-day persistence, and 27-day recurrence. 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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