Table 1 Modified Downs and Black checklist used to assess methodological quality of the included studies

Reporting

1

Is the hypothesis/aim/objective of the study clearly described?

2

Are the main outcomes to be measured clearly described in the Introduction or Methods section?

3

Are the characteristics of the patients included in the study clearly described?

4

Are the main findings of the study clearly described?

5

Does the study provide estimates of the random variability in the data for the main outcomes?

6

Have actual probability values been reported (e.g., 0.035 rather than < 0.05) for the main outcomes except where the probability value is less than 0.001?

External validity

7

Were the subjects asked to participate in the study representative of the entire population from which they were recruited?

8

Were those subjects who were prepared to participate representative of the entire population from which they were recruited?

Internal validity-bias

9

If any of the results of the study were based on ‘data dredging,’ was this made clear?

10

Were the statistical tests used to assess the main outcomes appropriate?

11

Were the main outcome measures accurate (valid and reliable)?