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'Dow Theory' doubters need to look at the data

A CSX train seen at the Norfolk Southern Bellevue Terminal in Bellevue, Ohio.
Justin Solomon | CNBC
A CSX train seen at the Norfolk Southern Bellevue Terminal in Bellevue, Ohio.

A debate playing out every day lately among Wall Street strategists is about whether investors should worry that shares of transportation companies, barometers of the health of the economy, are falling.

Don't fall asleep yet. The century-old "Dow Theory" may still matter in this world now dominated by advanced technology and social media stocks, history shows.