The net directional effect domain experts expect for a time series over the forecast horizon. Armstrong and Collopy (1993) classified them as classified them as growth, decay, opposing, regressing, or supporting forces. The typical assumption behind extrapolation is supporting, but such series are rare. Armstrong, Adya and Collopy (2001) discuss evidence related to the use of causal forces. 

• 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.
• Armstrong, J. S. & F. Collopy (1993), “Causal forces: Structuring knowledge for time-series extrapolation,” Journal of Forecasting, 12, 103-115.  (Full text)