Research guru and money manager James O'Shaughnessy forced many professional and amateur investors alike to rethink their investment beliefs when he published his 1996 bestseller, What Works on Wall Street. O'Shaughnessy back-tested 44 years of stock market data from the comprehensive Standard & Poor's Compustat database to find out which strategies work and which don't. To the surprise of many, he concluded that price-to-earnings ratios aren't the best indicator of a stock's value, and that small-company stocks, contrary to popular wisdom, don't as a group have an edge on large-company stocks.

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