Cita APA (7th ed.)
Ruffin, R. J., & Gregory, P. R. (1990). Principles of economics (4th ed.). Scott, Foresman.
Cita Chicago (17th ed.)
Ruffin, Roy J., i Paul R. Gregory. Principles of Economics. 4th ed. Glenview, Illinos: Scott, Foresman, 1990.
Cita MLA (9th ed.)
Ruffin, Roy J., i Paul R. Gregory. Principles of Economics. 4th ed. Scott, Foresman, 1990.
Atenció: Aquestes cites poden no estar 100% correctes.