The tracking signal is a measurement that indicates there may be bias in the forecast model. The tracking signal is the ratio of the cumulative forecast error to the mean absolute deviation (MAD). It measures how well the forecast is tracking the actual demand over time. A tracking signal of zero means that the forecast is perfectly accurate. A tracking signal within the range of -4 to +4 is considered acceptable. A tracking signal outside this range indicates that the forecast is consistently overestimating or underestimating the demand, which implies that there is bias in the forecast model. Bias is the tendency of a forecast to be consistently higher or lower than the actual demand. Bias can be caused by factors such as inaccurate data, inappropriate forecasting methods, or changes in demand patterns. References:

Managing Supply Chain Operations, Chapter 5: Demand Management and Forecasting, Section 5.2: Forecasting Methods, Subsection 5.2.3: Forecast Accuracy and Control

CPIM Exam Content Manual, Module 3: Demand, Section 3.2: Forecasting, Subsection 3.2.2: Forecasting Methods, Subsubsection 3.2.2.3: Forecast Accuracy and Control