However, telephone interviews can hardly be used in the case of a complex survey. The questionnaire must indeed be simple since it is impossible to show a written list of arguments to the respondent. Working out a questionnaire is a very important phase of the survey. The quality of the questionnaire is decisive for the pertinence of the survey. And asking the right questions, in an intelligible way, to a great number of people, is an art in itself.
Some surveys fall down through a too complex formulation which leads respondents astray.
For all these reasons, writting up a good questionnaire must be dealt with by competent professionals. The research institute is intellectually responsible for the questions asked to the respondents. But it often has to resist the pressure of its clients who, consciously or not, try to impose on the institute, questions written in a way that favours their point of view. In practise, the margin of error is considered to be equivalent to that of random surveys which can be calculated thanks to the Gauss law. The margin of error of a survey first depends on the number of respondents concerned.