Traditionally, for questions where it is legitimate for respondents to say they can’t answer, a “Don’t know” response has been added to the end of the answer list. For methods where a researcher is present “Don’t know” works satisfactorily. Often interviewers don’t even read out the “Don’t know” answer but it is there to use if the respondent genuinely can’t answer the question.
However, as online surveys have grown, “Not sure” has emerged as an alternative to “Don’t know”. Personally I favour “Not sure” over “Don’t know”. “Don’t know” can seem quite harsh. It can suggest that the respondent doesn’t know something they should. “Not sure” comes across as less critical. It suggests that respondents have considered the question but with the knowledge available to them they cannot choose an answer.
In the absence of an interviewer it is important that questionnaire designers do all they can to make surveys as respondent-friendly and engaging as possible. The softer “Not sure” option helps create a positive connection with respondents.