Table 4. Quality Assessment of the Nine Included Studies.

	Newcastle-Ottawa Scale*
Author	1	2	3	4	5A	5B	6	7	8	Total
Wang	Yes	Yes	Yes	Yes	Yes	Yes	Yes	No	Yes	8
Imran	Yes	Yes	Yes	Yes	No	No	Yes	No	Yes	6
Bae	Yes	Yes	Yes	Yes	No	Yes	Yes	No	Yes	7
Wu	Yes	Yes	Yes	Yes	No	No	Yes	No	Yes	6
Chung	Yes	Yes	Yes	Yes	No	No	Yes	Yes	Yes	7
Larsson	Yes	Yes	Yes	Yes	No	Yes	Yes	No	Yes	7
Lancaster	Yes	Yes	Yes	Yes	No	No	Yes	Yes	Yes	7
Kharaghani	Yes	Yes	No	Yes	Yes	Yes	No	Yes	Yes	7
Kim	Yes	Yes	Yes	Yes	No	Yes	Yes	Yes	Yes	8
Vollebregt	Yes	Yes	Yes	Yes	No	No	Yes	Yes	Yes	7
Qiu	Yes	Yes	No	Yes	Yes	No	No	Yes	Yes	6
Kurki	Yes	Yes	Yes	Yes	Yes	Yes	No	Yes	Yes	8

*1 indicates case definition or ascertainment of exposure; 2, representativeness of the cases or exposed cohort; 3, selection of controls or non-exposed cohort; 4, description of the control source or demonstration of outcome of interest in cohort; 5A, based on the most important factor to select and study controls; 5B, based on a second crucial factor to select and study controls; 6, assessment of exposure or outcome by blinded interview or record; 7, same method of ascertainment used for cases and controls or adequate follow-up period; 8, evenly distributed non-response rate or adequacy of follow-up of cohorts