Methodological quality.

Note: Please see Appendix S2 for an explanation of each methodological quality component. Briefly, each item was appraised as follows:1) Representativeness: a) truly representative, b) somewhat representative, c) selected group of users, d) no description of the derivation of the cohort.2) Selection: a) drawn from the same community as exposed cohort, b) drawn from a different source, c) no description.3) Ascertainment: a) secure record, b) structured interview, c) written self report, d) no description.4) Demonstration: a) yes, b) no.5) Comparability: a) study controls for age or gender, b) study controls for any additional factor, c) no control.6) Assessment: a) independent blind assessment, b) record linkage, c) self report, d) no description.7) Follow-up: a) yes, b) no.8) Adequacy: a) complete follow up, b) subjects lost to follow up unlikely to introduce bias, c) large loss to follow-up, d) no statement.