Well, that's of course a complicated question that depends on the product, the market, and situation. However, as I've noted previously, the questions you want answered often aren't the questions you should include in a survey. The reason is response bias.
In addition to some of the traditional advice for avoiding response bias, keep in mind that:
- The most interesting findings often come from correlating responses to more than one question.
- Including one or more open-ended (fill-in-the-blank or essay) questions allows for unanticipated answers and findings.
- You will cause respondents to answer randomly or systematically if you include too many questions in your survey.
- No matter how much you want them answered, it's pointless asking questions to which your respondents don't know the answers.
- In general, as a market researcher, you main goal is to understand customer psychographics, not just demographics.