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(explain distinction between prob measure and general measure, expand a bit on additivity) 

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A '''probability measure''' is a [[realvalued function]] defined on a set of events in a [[probability space]] that satisfies [[Measure (mathematics)measure properties]] such as ''countable additivity''.<ref>''An introduction to measuretheoretic probability'' by George G. Roussas 2004 ISBN 0125990227 [http://books.google.com/books?id=J8ZRgCNSwcC&pg=PA47 page 47]</ref> The difference between a probability measure and the more general notion of measure (which includes concepts like [[area]] or [[volume]]) is that a probability measure must assign 1 to the entire probability space.

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