Table 2.

Quality assessment of the studies included in this meta-analysis based on the Newcastle–Ottawa scale.

Authors	Selection				Comparability	Outcome			Total Score
	Representativeness of the exposed cohort	Selection of the non-exposed cohort	Ascertainment of exposure	Outcome of interest was not present at start		Ascertainment of outcome	Adequacy of duration of follow-up	Adequacy of completeness of follow-up
Diamond [21]	★	★	★	★	★★	★	★		8 (good)
Florschütz [22]	★	★	★		★	★	★		6 (good)
Morris [23]	★	★	★		★★	★	★	★	8 (good)
Richards [24]	★	★	★	★	★	★	★	★	8 (good)
Singh [25]	★	★	★	★	★	★	★		7 (good)
Werthel [26]	★	★	★		★	★	★	★	7 (good)
Everhart [27]	★	★	★	★	★★	★		★	8 (good)
Johansson [28]	★	★	★		★	★	★	★	7 (good)
	Selection				Comparability	Exposure
	Adequate definition of case	Representativeness of control	Selection of control	Definition of control		Ascertainment of outcome	Same method of ascertainment for cases and controls	Non-response rate
Nezwek [29]	★	★	★	★	★	★	★	★	8 (good)
Nagaya [30]	★	★	★	★	★	★	★		7 (good)