Analysis: Transports' slump could be harbinger of market trouble

Transportation stocks are closely watched because of their role in facilitating economic activity, and one tenet of "Dow theory," where the transportation and industrials indexes are looked at in tandem, is that when both start to fall, more weakness could be in the offing. 

Friday's drop in the transportation average came right after a record high on the index on Thursday, marking only the eighth time since 1900 the index lost more than 3 percent a day after a hitting a 52-week high, according to Sentiment Trader.

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