1. Babbie E. (1983). The Practice of Social Research. 3rd edn. Belmont, CA: Wadsworth Publishing Company
2. Babbie E. (1992). The Practice of Social Research, 6th edn. Belmont, CA: Wadsworth Publishing Company
3. Babbie E. (1995). The Practice of Social Research, 7th edn. Toronto, Wadsworth Publishing Company
4. Babbie E. (2001). The Practice of Social Research, 8th edn. Toronto, Wadsworth Publishing Company
5. Bogdan R., Ksander M. (1980). Policy data as a social process: A approach to quantitative data. Human Organization 39(4): 302–309