References
- Dillman , Don A. 1978 . Mail and Telephone Surveys: The Total Survey Method , New York : John Wiley & Sons .
- Moser , C. A. and Kalton , G. 1972 . Survey Methods in Social Investigation , New York : Basic Books .
- Snedecor , George W. and Cochran , William G. 1980 . Statistical Methods. , 7th Ed. , Ames : Iowa State Press .