Web Supplement 1. Electronic search strategy.

Web Supplement 2 . Quality assessment of individual trials was based on the risk of bias criteria of the GRADE Working Group. Quality grades of low, medium, or high were assigned to each trial based on the consensus opinion of the review authors.

Web Supplement 3. Funnel plots depict study precision as a function of strength of association.

PPE_1289_sm_WS1.doc27KSupporting info item
PPE_1289_sm_WS2-1.doc54KSupporting info item
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PPE_1289_sm_WS3f.tif134KSupporting info item
PPE_1289_sm_WS3g.tif152KSupporting info item

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