Missing answers occur for several reasons, including refusal or inability to answer a particular question, and cases where the question is not applicable to the respondent.
Gall atebion fod ar goll am sawl rheswm, gan gynnwys gwrthod neu fethu ateb cwestiwn penodol, ac achosion pan nad yw'r cwestiwn yn berthnasol i'r ymatebydd.