Checkboxes don't have a Yes/No option, only checked/unchecked; it's then up to you to (a) ensure the instructions are clear as to what the options represent; and (b) design the form in such a way that the boxes will only remain checked/unchecked through a deliberate decision, not inadvertently. The alternative is to use a dropdown that starts off with a 'null' value (perhaps as part of the question itself) and allows the user to choose between the Yes & No answers.

To conditionally output content as a result of a content control selection, a Document_ContentControlOnExit macro is required. A search of these forums will turn up plenty of examples of their use. Note also that content controls mostly don't work properly on Mac versions of Word.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]