Table 2 Summary of quality assessment with the Down and Black Checklist

Reporting

Aims are clear

1

1

1

Main outcomes are clear

1

1

1

Characteristics of the patients are clear

0^a

1

1

Confounders in each group clear

1

1

1

Main findings are clear

1

1

1

Estimates of the random variability

1

1

1

Losses to follow-up

0

0

1

P values reported

1

1

1

External validity

Representative selection process

0^b

0b

0

Representative sample

0

1

0

Internal validity

Data dredging

1

1

1

Different lengths of follow-up

1

1

1

Statistical tests

1

1

1

Accurate main exposure measurement

0

1

0

Accurate outcome measurement

1

0

1

Confounding

All participants from the same population

1

1

1

Participants selected simultaneously

1

1

1

Adjustment for confounding

0

0

0

Losses

0

0

0

Power

Sample power calculation

0

0

0

Score

12^c (16^d)

14 (19)

14 (19)