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This paper is based on a Cochrane Review published in The Cochrane Library 2011, Issue 51 (see http://www.thecochranelibrary.com for information). Cochrane Reviews are regularly updated as new evidence emerges and in response to feedback, and The Cochrane Library should be consulted for the most recent version of the review.
Competing interests None.
Provenance and peer review Not commissioned; internally peer reviewed.
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