Table 2. Methodological quality of included studies.

Newcastle–Ottawa Scale (NOS) Quality Assessment Table
Study	Selection	Comparability	Exposure/Outcome	Total Star
Dong-liang Chen [13]	++++	+	+++	8
Lei Ma [19]	++++	+	++	7
Peng Song [22]	++++	+	+++	8
Wei Wei [23]	+++	+	+++	7
G.-L. LI [15]	++++	+	+++	8
Xiaoliang Wu [24]	++++	+	+++	8
Yichao Mo [21]	++++	+	++	7
Weijie Ma [20]	++++	+	+++	8
Reiko Kobayashi [14]	++++	+	++	7

A star system was used to allow a semiquantitative assessment of study quality. A study could be awarded a maximum of 1 star for each numbered item within the selection and exposure categories. A maximum of 2 stars could be given for comparability. The NOS ranged from 0 to 9 stars. Studies achieved 7 were considered as high-quality ones, 4 to 6 stars were medium-quality studies, and < 4 stars were poor-quality studies.