A "skipped" question is one that has not been answered but has a Question Number that is lower than the highest Question number that has been answered.

An "Unanswered Question" is a question with a higher Question Number than the highest Question Number that has an answwer.

Of course, you may have already thought of the question that I came up with when presented with this problem, "How do I know whether Question N+1, where N is the highest Question Number with an answer, is an unanswered vs a skipped question?"

The Skipped Questions will have a NULL because there is no answer marked (and, therefore, no value to record). They will not have a value of -1 because a) they have no answer and b) there is nothing to preclude their being True/False, Multiple Choice (with alpha choices) or even essay questions.