I think that adding “don’t know/don’t want to answer” in questionnaires is a good catch-all phrase.
Sometimes, people have issues with the simplest questions.
You ask their phone, but they prefer leaving only an email address.
You ask for their name, but they prefer the questionnaire to be anonymous, without using false data (like giving a false name).
You ask “For how long have you lived in City X?”, but you miss some important detail which makes the question hard to answer.
I’m all in favor of adding “don’t know/don’t want to answer” to questionnaires.
