Table 2.

Quality assessment of cohort studies

Studies	Jiang et al.	Singh et al.	Schwarzkopf et al.
Representativeness of the exposed cohort	Low risk	Low risk	Low risk
Selection of the non-exposed cohort	Low risk	Low risk	Low risk
Ascertainment of exposure	Low risk	Low risk	Low risk
Outcome not present at start	Low risk	Unclear bias	Unclear bias
Adjustment for primary and secondary factors	Low risk	Low risk	Low risk
Assessment of outcome	Low risk	Low risk	Low risk
Was follow-up long enough for outcomes to occur	Low risk	Low risk	Unclear bias
Adequacy of follow-up of cohorts	Low risk	Low risk	Unclear bias
Quality	High quality	Moderate quality	Moderate quality