Table 3.

Quality assessment

Study	Selection				Comparability of cohorts	Outcome
	Representativeness of the exposed cohort	Selection of the non exposed cohort	Ascertainment of exposure	Outcome not present at start		Assessment of outcome	Follow-up long enough?	Adequacy of F-U	Total scores
Kenig et al14	✩	✩	–	✩	✩✩	✩	✩	✩	8
Tan et al18	✩	✩	–	✩	✩✩	–	✩	✩	7
Lee et al13	✩	✩	✩	✩	–	–	✩	✩	6
Pujara et al19	✩	✩	–	✩	–	–	✩	✩	5
Kristjansson et al17	✩	✩	✩	✩	✩✩	✩	✩	✩	9
Kristjansson et al16	✩	✩	–	✩	✩✩	✩	✩	✩	8

Notes: The Newcastle-Ottawa Scale (NOS) evaluates each study on eight items grouped into three categories including the study population selection, comparability, and outcome. A study can be awarded one star for each item within the selection and outcome categories and two stars for comparability. One star refers to a score of 1 and studies with a total score of ≥7 were considered as high quality studies.

Abbreviation: F-U, follow-up.