How long should a questionnaire be?

A questionnaire should be as short as possible, while ensuring the researcher is able to gather valid and reliable answers to all questions asked of each respondent.

Questionnaire length, often referred to as length of interview or “LOI,” is typically measured by the average time it takes a respondent to complete the entire questionnaire when it is administered in a similar context to that in which it will ultimately be fielded.

There is no objective limit on how long a questionnaire may be, nor any absolute maximum number of questions it may contain. The mental burden a questionnaire places on a respondent will vary not only based on the individual respondent but also based on how the questionnaire itself has been designed, what kinds of questions it contains, how and when it is administered, and many other factors.


What should you consider when writing a questionnaire?

While there are no hard-and-fast rules governing questionnaire length, there are a few general guidelines good researchers tend to keep in mind when designing questionnaires for common market research contexts. These guidelines take into account the fact that as questionnaire length increases, the ability of respondents to provide valid and reliable answers to each question decreases.

Simultaneously, response validity (i.e., data quality) eventually deteriorates to the point that any further questions asked of the respondent are not worthwhile, since the individual’s attention span and ability to provide valid answers has been totally exhausted.



Common rules of thumb

Over decades of academic and professional research practice, researchers have found that following the rules of thumb below for length of interview generally ensures questionnaires are kept sufficiently short to collect high-quality responses from all respondents to all questions, while still providing sufficient room for thorough questioning on most topics of interest to researchers and their clients.

In general, questionnaire length (average LOI) should be limited to:

  • 10 to 15 minutes for self-administered online surveys
  • 20 to 30 minutes for semi-structured telephone interviews
  • 30 to 60 minutes for in-depth, structured telephone or face-to-face interviews

The benefits of pretests

In research contexts where the above rules of thumb are not relevant or applicable, the best way to assess whether a given questionnaire is “too long” or not is simply to (pre)test it before it is actually fielded – i.e., by administering it to a small sample of test respondents and gauging their ability to provide high-quality responses to every question it contains within a reasonable timeframe. If your pre-testers can complete the entire questionnaire in an identical or similar context to that in which you are ultimately planning to field it, then chances are, most of your actual target respondents will be able to do the same.

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