Mean size of errors for the forecast of the local K-index. The plots include results for the SIDC forecast and the numerical models persistence, recurrence and corrected recurrence of 14 and 27 days. The left vertical axis displays the error (colored bars) and the right vertical axis the skill score (crosses) for each of the forecast models with respect to the persistence model as reference model. The mean errors (red bars, Eq. (3)) as well as the mean absolute errors (green bars, Eq. (4)) are displayed.