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My thought was either your mental model a) is that yes/no is exactly that (bistate), or b) that "I don't know" is an acceptable answer (tristate). Under this point of view it makes no sense to offer both at the same time. It was not intended as a shortcut for avoiding thinking about the problem.
If I had it to do again I would not offer bistate, instead using default values to map "I don't know" to "No".
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