Incomplete response options
The question must be presented to respondents as intelligibly, clearly and unambiguously as possible. The same applies to response options that must enable the respondent to express his or her opinion without difficulty. That way we can check if we included all options with just one question (e.g. what is your religion?).
If we are not completely sure we can add the option "OTHER". Similarly, we add options "I DO NOT KNOW" or "I DO NOT WISH TO ANSWER" when there is a chance that respondents are not familiar with the issue (e.g. policy XXX) or do not wish to provide an answer (e.g. household income). Of course, it is important to carefully consider if such additional answer options are really necessary, or else respondents that would have answered the question may instead avoid it needlessly.
If these options are not really necessary, we should avoid them.