Forces that reinforce the historical trend. Real-world examples of supporting forces are difficult to find because information about the trend in a series is assumed to be the dominant factor affecting behavior, and other factors are unimportant. Supporting forces might occur over specific periods for sales of fashion crazes or fad items, inflation, or for market prices such as for real estate or for internet stocks in the late 20^th century. See Armstrong, Adya and Collopy (2001).

• Armstrong, J. S., M. Adya & F. Collopy (2001), “Rule-based forecasting: Using judgment in time-series  extrapolation,” in J. S. Armstrong (ed.), Principles of Forecasting. Norwell, MA: Kluwer Academic Press.