Fig. 1

Flowchart of the systematic literature search. The search terms yielded 3052 articles where 2684 were excluded on review of titles and abstracts due to lack of relevance leaving 368 articles. 82 of these articles were not retrieved as 74 were duplicates and 8 were inaccessible in our library or via interlibrary loans. Of the remaining 286 articles, 246 were excluded as 238 did not meet inclusion criteria and 8 had insufficient information to calculate effect sizes despite attempting to contact the corresponding authors. The remaining 40 articles were included for final review.