7.1 The Three-variable Linear regression Model three-variable prf: nonstochastic form: E(Y=B+B2X2t+B3 (71) stochastic form: YtB+B2x2t+B3X3+ut (7.2) E(YD+ut B2, B3partial regression coefficients, partial slope coefficient B the change in the mean value of Y, e(Y), per unit change in X2 holding the value ofx constant B3: the change in the mean value of Y per unit change in X3, holding the value of x2 constant.7.1 The Three-variable Linear Regression Model three-variable PRF: nonstochastic form: E(Yt )=B1+B2X2 t+B3X3t (7.1) stochastic form: Yt=B1+B2X2 t+B3X3t+ut (7.2) = E(Yt )+ut B2 , B3～partial regression coefficients, partialslope coefficient B2: the change in the mean value of Y, E(Y), per unit change in X2, holding the value of X3 constant. B3 : the change in the mean value of Y per unit change in X3 , holding the value of X2 constant