The invention provides methods and apparatus, including computer program products, for correcting outlier values in a series of values representing a predetermined numerical parameter over time. For predetermined time interval with a beginning (b) and end (e) point of the time series, an ex-post forecast time series (s_ep) is calculated using the historical time series (s_h) and a predetermined model function (f). Lower and upper tolerance limit lines are defined using (s_ep) and on a quality function (qf) over the time interval. Beginning at (b), (s_h) is corrected by replacing the first value outside of the tolerance lines by a predetermined value inside the tolerance lines. The (s_ep) is recalculated using (s_h) and (f). The tolerance lines are redefined using the recalculated (s_ep) and (qf) that takes only values where the last outlier value has been replaced. These steps are repeated until all (s_h) outlier values have been replaced.
