Trend-Adjusted Exponential Smoothing
The exponential smoothing approach discussed above is an appropriate forecasting technique, if the time series exhibits a horizontal pattern (i.e. No trend) with random fluctuations. However, if the time-series exhibits trend, forecasts based on simple exponential smoothing will lag the trend. In such cases, a variation of simple exponential smoothing called the trend-adjusted Exponential smoothing can be used as a forecasting technique. "The trend-adjusted forecast (TAF) has two components:
- A smoothed error
- A trend factor
TAFt = St-1 + Tt-1 , where
St-1 = Previous period smoothed forecast
Tt-1 = Previous period trend estimate
TAFt = Current period's trend-adjusted forecast
St = TAFt + (At - TAFt)
Tt = Tt-1 + (TAFt - TAFt-1 - Tt-1), where and are smoothing constants
In order to use this method, one must select values of and (usually through trial and error) and make a starting forecast and an estimate of the trend" (Stevenson, 2005).