Table 2 Quality assessment of the included studies
Study | Representativeness of the exposed cohort | Selection of the unexposed cohort | Ascertainment of exposure | Outcome of interest not present at start of study | Control for important factor or additional factor | Outcome assessment | Follow-up long enough for outcomes to occur | Adequacy of follow-up of cohorts | Total quality scores |
|---|---|---|---|---|---|---|---|---|---|
Hao et al. [20] | ☆ | ☆ | ☆ | – | – | ☆ | ☆ | ☆ | 6 |
He et al. [23] | – | ☆ | ☆ | – | – | ☆ | ☆ | ☆ | 5 |
Jia et al. [15] | – | ☆ | ☆ | – | – | ☆ | ☆ | ☆ | 5 |
Kim et al. [35] | – | ☆ | ☆ | – | – | ☆ | ☆ | ☆ | 5 |
Kong et al. [17] | – | ☆ | ☆ | – | – | ☆ | ☆ | ☆ | 5 |
Ono et al. [16] | – | ☆ | ☆ | – | – | ☆ | ☆ | ☆ | 5 |
Wang et al. [25] | ☆ | ☆ | ☆ | – | – | ☆ | ☆ | ☆ | 6 |
Yu et al. [26] | – | ☆ | ☆ | – | – | ☆ | ☆ | ☆ | 5 |