Will add language in Questionnaire to note that systems SHOULD use controls (check boxes, radio buttons, etc.) that distinguish between non-answered questions and those that a specific answer has been chosen. I.e. If a question has a boolean answer or is a "pick one" of a set of choices (either of which might reasonably be represented by a radio button and the first of which could be represented by a single check-box), it should be possible to distinguish "user hasn't selected an option" and it should be possible for the user to return to that state. This may mean that default XHTML controls may not necessarily provide the desired behavior. If using default XHTML controls, one option would be to have a set of radio buttons where the first (and default) option is "no selection made". Selecting that option would correspond to omitting an answer for that Questionnaire.item. Note that this concern only applies if the question is optional or doesn't have a default answer. Will also add guidance that enableWhen expressions need to differentiate between "no answer" and "false" for boolean elements.