Additional file 1.

Quality. DOC file which describes the components of the Newcastle-Ottawa study quality scoring system, shows the scores allocated to each study, and for some scores gives the reason the study scored as negative. Scores relate to eight items - 1: “representativeness of the exposed cohort”, 2: “selection of the non-exposed cohort”, 3: “ascertainment of exposure”, 4: “demonstration that the outcome of interest was not present at start of the study”, 5: “comparability of the cohorts on the basis of design or analysis”, 6: “assessment of outcome”, 7: “was follow-up long enough for outcomes to occur”, and 8: “adequacy of follow up of cohorts”. Apart from item 5, which is scored as 0, 1 or 2, each item is scored as 0 or 1, so the total possible score for a study is 9.

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Fry et al. BMC Cancer 2012 12:498   doi:10.1186/1471-2407-12-498