Cooper, D. R., & Schindler, P. S. (1998). Business research methods (6th ed.). Irwin/McGraw-Hill.

Cooper, Donald R., and Pamela S. Schindler. Business Research Methods. 6th ed. Boston: Irwin/McGraw-Hill, 1998.

Cooper, Donald R., and Pamela S. Schindler. Business Research Methods. 6th ed. Irwin/McGraw-Hill, 1998.