IF and only if one is willing to say "As the number of highly-rated reply nodes to a question increases, the question can be considered increasingly answered," then you could build an 'answeredness' value accordingly.

Measure the "goodness" of each direct reply by how high it is ranked when compared with the highest and lowest ranked nodes, average "goodness"es, and that is the answeredness.

This is only if you're willing to be very patient and wait for the answers to filter in. Perhaps that's the whole goal of the "zombie questions" idea, but I find it a little distasteful, if only because it would encourage waiting. ("Let's see if someone else answers it!") It would also provide no way to be sure that subsequent replies didn't answer the question (although you could factor them in at a reduced rate, I suppose).

Lots of work, seemingly little payoff, IMHO.

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You are what you think.