Plenty of theories are known because they are useful, and it is up to you to discern which ones may be worth your time and fit your situation and investment or analysis style.
There’s always merit to any theory which has been put through rigorous statistical tests. However, keep in mind that as with any other statistical inferences, an event with probability zero sometimes happens (Black Swans), and an event with probability one sometimes doesn’t.
It is well known, for example, that the market has predicted fourteen out of the last six recessions; this is a jab at the apparent inability of bear markets to indicate or predict recessions in the economy as a whole.
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