2.3.Classification of Forecasts -Objective Causal Model The method of least squares is most commonly used for finding estimators of these constants. Assume we have the past data (xi,y),i=1~n;and the causal model is simplified as Y=a+bX.Define g(a,b)=∑y-(a+bx,)P i=l as the sum of the squares of the distances from line a+bX to data points yi.Causal Model The method of least squares is most commonly used for finding estimators of these constants. Assume we have the past data (xi, yi), i=1~n; and the causal model is simplified as Y=a+bX. Define 2 1 ( , ) [ ( )] n i i i g a b y a bx     as the sum of the squares of the distances from line a+bX to data points yi. 2.3. Classification of Forecasts - Objective