For those who are interested learning more about the science behind some of our analytics, this paper, Time series Forecasting using Holt-Winters Exponential Smoothing (Kalekar 2004), is an excellent starting point. Unfortunately, there are a number of minor typos in the formulas for the initial conditions. If you are implementing your own Holt-Winters method, you want to consult alternative sources, e.g. Wikipedia, to find corrected versions.
Many industrial time series exhibit seasonal behavior, such as demand for apparel or toys. Consequently, seasonal forecasting problems are of considerable importance. This report concentrates on the analysis of seasonal time series data using Holt-Winters exponential smoothing methods. Two models discussed here are the Multiplicative Seasonal Model and the Additive Seasonal Model.