Study Quality
Two reviewers assessed the quality of all studies based on a Newcastle-Ottawa Quality Assessment Form for Cohort Studies in table 2 (supplement 3).21 The maximum score was 9, and the minimum score was 0. The score of 7 or more indicated high quality, a score of 4 to 6 indicated moderate quality, and a score 3 or less indicated low quality. The reviewers resolved the disagreement through discussion until they reached consensus.