Table 2.

Newcastle–Ottawa scale assessment of the quality of the studies

Study	Selection				Comparability based on design or analysis	Exposure			Total scores	Qualitya
	Case definition adequate	Representativeness of the cases	Selection of controls	Definition of controls		Ascertainment of exposure	Same method of ascertainment for cases and controls	Nonresponse rate
Tse et al (2008)23	⋆b	⋆	⋆	⋆	⋆	⋆	⋆	✰	7	High
Zheng et al (2009)24	⋆	⋆	⋆	⋆	⋆⋆	✰	✰	✰	6	Medium
Fan (2013)25	⋆	⋆	⋆	⋆	⋆⋆	⋆	⋆	✰	8	High
Xu et al (2014)26	⋆	⋆	⋆	✰	⋆	⋆	⋆	✰	5	Medium
Ding et al (2016)27	⋆	⋆	⋆	✰	⋆⋆	⋆	✰	✰	6	Medium
Xu et al (2017)28	⋆	⋆	⋆	✰	⋆⋆	⋆	⋆	✰	7	High
Kundu et al (2018)29	⋆	⋆	✰	✰	⋆⋆	⋆	⋆	✰	6	Medium

Notes:

^aThe quality is assessed by the score as follows: ≥7, high quality; ≥5 to <7, medium quality; <5, low quality.

^bCells with a star indicate that the corresponding item is addressed satisfactorily in the publication in question.