Table 1 Aspects of quality reviewed

1. Did the review answer a clearly focused question?

2. Was the cohort recruited in an acceptable way?

3. Was the exposure accurately measured to reduce bias?

4. Has the study identified any confounding factors?

5. Has the study taken into account all of the confounding factors in the analysis

6. Was the outcome accurately measured to minimise bias?

7. How precise are the results?

8. How generalizable are the results?