1. Carl F. Christ,Econometric Models and Methods. New York: John Wiley and Sons, 1966, p. 387.
2. Economics Courses Offered and Required in Two-year Colleges
3. J. Johnston,Econometric Methods. New York: McGraw-Hill, 1963, pp. 201–203.
4. Lawrence R. Klein,An Introduction to Econometrics. Englewood Cliffs, N.J.: Prentice-Hall, Inc., 1962, p. 101.