Methods
We followed the Cochrane Collaboration’s method for this review.15 We did comprehensive, systematic literature searches of PubMed (from 1966 to 31 May 2012), and Embase (from 1974 to 31 May 2012). We developed separate search strategies consisting of a combination of free text words (tw), words in titles/abstracts (tiab), and medical subject headings (mesh) for exposure, participants, and study design; we then combined these by using “AND” (see box). We placed no language or publication restrictions. We adapted the PubMed search strategy for Embase. We screened reference lists of identified studies and published reviews for additional studies.