Assessment of study quality.

For cohort studies: 1, representativeness of exposed cohort; 2, selection of the nonexposed cohort; 3, ascertainment of exposure; 4, outcome of interest not present at start; 5a, cohorts comparable on basis of main factor; 5b, cohorts comparable on any additional factor; 6, assessment of outcome with independency; 7, follow-up long enough for outcomes to occur; 8, complete accounting for cohorts or subjects lost to follow-up unlikely to introduce bias.Assessment of study quality.