APA

Ruffin (Roy J), Gregory (Paul R), . (1993). Principles of Economics. New York: Harper Collins College Publishers.

Chicago

Ruffin (Roy J), Gregory (Paul R), . 1993. Principles of Economics. New York: Harper Collins College Publishers.

Harvard

Ruffin (Roy J), Gregory (Paul R), . (1993). Principles of Economics. New York: Harper Collins College Publishers.

MLA

Ruffin (Roy J), Gregory (Paul R), . Principles of Economics. New York: Harper Collins College Publishers. 1993.