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. 2019 Feb 23;86(3):370–375. doi: 10.1016/j.bjorl.2019.01.003

Table 1.

Basic information of the 4 studies in this meta-analysis.

Year First author Country Ethnicity Source of the subjects Cases Controls
2018 Yang China Asian Steel factory 340 339
2016 Yang China Asian Steel factory 283 281
2013 Li China Asian Chemical fiber company 340 356
2016 Xu China Asian Steel factory 236 235