Disturbances in the linear model, estimation and hypothesis testing

When plotting the ob served pairs (x l' YI)' (x ' Y2)' . , (x , Y n) into a diagram with x 2 n measured along the horizontal axis and y along the vertical axis it rarely occurs that all points lie on a straight line. Generally, no b 1 and b exist such that Yi = b + b x for i = 1,2, .