4Shewhart W A. Economic Control of Quality of Manu-factured Product [M]. Van Nostrand: ASQ QualityPress,1931.
5Shewhart W A. On the measurement of a physical quan-tity whose magnitude is influenced by primary causes be-yond the control of the observer and on the method ofdetermining the relation between two such quantities[J],Proc Natl Acad Sci U S A,1922,8(8) :248-251.