Beta Is Four-Letter Word in Stock Tale Told by Expedia, Facebook

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Beta has become a bad word on Wall Street, just in time for earnings season.

The second letter of the Greek alphabet is used as shorthand for a measurement of volatility in a security. Put simply, if a benchmark index rises 1 percent you can expect a high-beta stock to rise significantly more than 1 percent. If the benchmark falls more than 1 percent, look out below.