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Murray R. Spiegel, Theory and Problems of Statistics, Schaum's Outline Series (2nd ed., NY: McGraw-Hill, 1988). The formula is on p. 158 of the first edition (NY: Schaum Publishing Co., 1961).

While the Schaum's book is a very convenient reference, it's not so easy on the beginner; for that, try Larry Gonick and Woollcott Smith's Cartoon Guide to Statistics (NY: HarperPerennial, 1993).