Table 3. Risk of bias assessment.
| STUDY | SELECTION | COMPARABILITY | EXPOSURE |
| Riefkohl, 2017 | ✵ | ✵✵ | |
| STUDY | SELECTION | COMPARABILITY | OUTCOME |
| Yang, 2015 | ✵✵ | ✵✵ | ✵✵✵ |
The more ✵ there are, the less risk of bias.
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