As with interviewing or focus groups, when designing a questionnaire start with easy questions which respondents will enjoy answering, thus encouraging them to continue filling in the questionnaire. If you begin with complex questions which need long responses, your respondents will be less likely to fill in the form. If you’re constructing a combined questionnaire, keep your open-ended questions for the end as, once someone has spent time completing the rest of the questionnaire, they are more likely to continue with those questions which take a little more effort to complete.