An extension of Holt's exponential smoothing method that includes seasonality (Winters 1960). This form of exponential smoothing can be used for less-than-annual periods (e.g., for monthly series). It uses smoothing parameters to estimate the level, trend, and seasonality. An alternative approach is to deseasonalize the data (e.g., via Census Program X-12), and then use exponential smoothing. There is little evidence on which seasonality procedure is most accurate. See state-space model.