Hidden questions are kept for revision purposes, along with any data that has been collected, but are not displayed on the public-facing project site or in results reports.

To hide a question, go to the question's settings:  (view results & edit)

Click the Options drop down menu

Click Hide Question (make inactive)

The hidden question will be visible only to administrators, and will appear condensed below any active questions. 

Related:
How to un-hide a question
How to archive a question
how to un-archive a question
How to delete a question

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