Table 2 Quality assessment of included cohort studies

 Representativeness of the exposed cohort

1

1

1

 Selection of the non-exposed cohort

0

1

1

 Ascertainment of exposure

1

1

1

 Demonstration that outcome of interest was not present at start of study

1

1

1

 Comparability of cohorts based on the design or analysis

2

2

2

 Assessment of outcome

1

1

1

 Was follow-up long enough for outcomes to occur?

1

1

1

 Adequacy of follow-up of cohorts

1

1

1

Total

8

9

9