Singleton, R. A., & Straits, B. C. (2005). Approaches to social research (4th ed.). Oxford.

Singleton, Royce A., and Bruce C. Straits. Approaches to Social Research. 4th ed. New York: Oxford, 2005.

Singleton, Royce A., and Bruce C. Straits. Approaches to Social Research. 4th ed. Oxford, 2005.