Theil's Statistic is an accuracy measure that compares the accuracy of a forecast model to the actual model. Theil's uses the actual value of the last time period as the forecast. The closer Theil's Statistic is to zero the more accurate the forecasting model. For instance, if Theil's Statistic is equal to 1, the forecast is completely inaccurate. If it is equal to 0 then the forecasting model is a perfect fit.