As mentioned before, it is very important to pretest ("pilot") the questionnaire. The questionnaire should be piloted with a similar group of people to your intended subjects. The aim of a pilot survey is to obtain estimates about the expected response rates, data quality, the validity and comprehensibility of the questionnaire (1).
It allows highlighting problems such as inappropriate questions or ambiguity before starting the real survey. The effects of an alternative wording should be tested as well. A pilot questionnaire should also include questions about the overall impression on respondents and interviewers in order to include their comments. It might be useful as well to ask non-respondents for the reasons for not participating.
Often the pilot study leads to several amendments before the survey starts. Therefore, there should be sufficient time allowed for this phase in the orginal time schedule ofthe study.
1. Silman AJ and Macfarlane GJ: Epidemiological studies. 2nd edition 2001, Cambridge. Pp 138-141