If Pr(crash|drop) is not quite large, then Pr(smallish decline|any decline) should be reasonably big. Each observation of a drop without a crash is (some) evidence against the antecedent.
Each time you strike a match without blowing up might be evidence that you're not in a building full of gasoline, but if you see the gasoline, the match-striking evidence doesn't weigh nearly as heavily.
I was slightly more specific in another question chain about what I meant by slump, and a 10% drop isn't quite what I had in mind (neither is today's slump, which could rally again), which was more along the lines of a recession.
To clarify my prediction: I expect at least a recession. Given a recession, I expect a depression. If we don't get a recession (which would surprise me somewhat), the absence of a depression won't surprise me, but if we do get a recession, the absence of a depression will surprise me a lot.
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