Table 2. Study Assessment According to Newcastle-Ottawa Rating Scale^a.

Source	Study Design	Selection	Comparability	Exposure/Outcome	Total Score
Forty et al,14 2014	Cross-sectional	NA	NA	★★	2
Huang et al,13 2019	Prospective cohort	★★★★	★	★★★	8
Kilbourne et al,15 2004	Cross-sectional	★★	★	★★★	6
Lin et al,10 2014	Retrospective cohort	★★★★	★★	★★	8
Marras et al,11 2016	Retrospective cohort	★★★★	★	★★★	8
Nilsson et al,12 2001	Prospective cohort	★★★★	★★	★★★	9
Smith et al,16 2013	Cross-sectional	★★★★	★★	★★★	9

Abbreviation: NA, not applicable.

^aRated on a scale of 1 star to 9 stars.