Abstract
This review was withdrawn from The Cochrane Library Issue 3, 2011 because the results are now out‐of‐date and we realise that the information therein (which has the potential for being used for policy decisions) could be used inappropriately. Please refer to the BMJ article www.bmj.com/content/342/bmj.c7258.full?sid=dc37372f‐dde0‐455c‐8725‐dd188f1fba4d for more information.
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