A key cash flow metric makes these stocks a smart buy, Ned Davis Research says

Monster Beverage Corp. drinks.
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The best strategy for picking stocks over the past three decades may be at the peak of its powers right now, according to Ned Davis Research.

Brian Sanborn, the firm's senior vice president for wealth management, said in a note that the ratio of a company's free cash flow to its enterprise value has been the best factor for picking stocks since at least 1988.

Investing in the best stocks by this metric has given a positive return more than 64% of the time over the last 33 years, slightly more than the S&P 500, according to the firm.

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