Inverted Initial unemployment claims.

Watch the cross here. Note the extremely close match to the overall stock market, and how this LEADS recessions. This data has a few small false signals going back to the 60's, but it has properly led every recession regardless of that fact. If you combine this signal with something simple like yield curve inversion, you would get one of the easiest and best market timing indicators out there. Pretty simple really.
Beyond Technical AnalysisChart PatternsICSATrend AnalysisunemploymentUNRATE

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