Final answer:
A bias of -10 in forecasting indicates the method is consistently forecasting lower than actual values, meaning it is underestimating.
Step-by-step explanation:
If a method has a bias of -10, it means that the method is consistently off by -10 units in its forecasts compared to the actual values it is trying to predict. In this context, a negative bias indicates that the method's forecasts are systematically lower than the actual values. Therefore, a bias of -10 implies that the method is underestimating.