Telephone surveys are the most common way of gathering information from respondents.
Sample size depends on the purpose and the design of the survey. The interviewers use a computer software which helps to ensure standard and consistency in each telephone interview. The interviewers read out questions, and sometimes answer alternatives, verbatim off the computer screen and enter the answers into the computer simultaneously. The software leads the interviewer through the questionnaire, making sure that appropriate questions are asked or skipped when that applies. Interviewers receive training and are given thorough instructions on each project. They abide by strict work rules on anonymity and privacy.
In telephone surveys, the usual aim is to achieve a response rate around 65-70%. The surveys vary in length and duration, depending on how the type of sample and the length of questionnaire. For example, an estimated 2-3 weeks is needed to gather data for a common 1500 person sample from the national register with an average length questionnaire (25-30 questions).
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