Table 2 The Newcastle–Ottawa scale (NOS) was used for assessing the quality of included studies
Items | He et al. [7] | Gao et al. [5] | Yin et al. [15] | Wu et al. [9] | Liu et al. [13] | Xu et al. [8] | Xu et al. [14] |
|---|---|---|---|---|---|---|---|
Is the case definition adequate | ★ | ★ | ★ | ★ | ★ | ★ | ★ |
Representativeness of the cases | ★ | ★ | ★ | ★ | ★ | ★ | ★ |
Selection of controls | ★ | ★ | – | ★ | ★ | – | ★ |
Definition of controls | ★ | ★ | ★ | ★ | ★ | ★ | ★ |
Comparability | |||||||
Study controls for the most important factor | ★ | ★ | ★ | ★ | ★ | ★ | ★ |
Study controls for any additional factor | ★ | – | ★ | ★ | – | ★ | – |
Exposure | |||||||
Ascertainment of exposure | ★ | ★ | ★ | ★ | ★ | ★ | ★ |
Same method of ascertainment for cases and controls | ★ | ★ | ★ | ★ | ★ | ★ | ★ |
Nonresponse rate | – | – | – | – | – | – | – |
Total scores | 8 | 7 | 7 | 8 | 7 | 7 | 7 |