(a).
| Case-control study | Selection | Comparability | Exposure | Total | |||||
|---|---|---|---|---|---|---|---|---|---|
| Definition adequate | Representativeness of the cases | Selection of controls | Definition of controls | Control for important factor or additional factor | Ascertainment of exposure | Same method of ascertainment for cases and controls | Nonresponse rate | (0–9) | |
| Wei-Sheng Lee-2017 [16] | ∗ | ∗ | ∗ | ∗ | ∗∗ | ∗ | ∗ | 8 | |
| Wei-Sheng Lee-2016 [17] | ∗ | ∗ | ∗ | ∗ | ∗∗ | ∗ | ∗ | 9 | |
Note: a “star (∗)” system of the Newcastle-Ottawa Scale (NOS) has been developed for the methodological quality assessment: each study can be awarded a maximum of one star for each numbered item within the selection and exposure categories, while a maximum of two stars can be given for the comparability category.