1. American Statistical Association. 2004a. ASA series: What is a survey?Designing a questionnaire, [On-line]. Available:http://www.amstat.org/sections/srms/brocures/survwhat.html
2. American Statistical Association. 2004b. ASA series: What is a survey?How to conduct pre-testing, [On-line]. Available:http://www.amstat.org/sections/srms/brocures/survwhat.html
3. American Statistical Association. 2004c. ASA series: What is a survey?How to plan a survey, [On-line]. Available:http://www.amstat.org/sections/srms/brocures/survwhat.html.
4. American Statistical Association. 2004d. ASA series: What is a survey?What are focus groups?, [On-line]. Available:http://www.amstat.org/sections/srms/brocures/survwhat.html.