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.