Table 1

Risk of bias for cohort studies as assessed with the NOS

 Zemper 200340 Kontos et al, 200637 Chrisman et al, 201935 Tsushima et al, 201936 Emery et al, 201038 Emery et al, 201139 Black et al, 201634 Selection Representativeness of the exposed cohort ∗ ∗ ∗ ∗ ∗ ∗ ∗ Selection of the non-exposed cohort ∗ ∗ ∗ ∗ ∗ ∗ ∗ Ascertainment of exposure ∗ ∗ ∗ ∗ ∗ Demonstration outcome not present at start ∗ ∗ ∗ ∗ ∗ ∗ Comparability Comparability of cohort on basis of design or analysis ∗ ∗∗ ∗∗ ∗∗ ∗∗ ∗∗ ∗∗ Outcome Assessment of outcome ∗ ∗ ∗ ∗ ∗ ∗ ∗ Was follow-up long enough for outcomes to occur? ∗ ∗ ∗ ∗ ∗ ∗ ∗ Adequacy of follow-up cohorts ∗ ∗ ∗ ∗ ∗ ∗ Total stars 7 9 8 7 9 9 9
• Maximum total score is 9. A study can be awarded a maximum of one star for each numbered item within the selection and outcome categories. A maximum of two stars can be given for comparability.30 A high score indicates a low risk of bias.

• NOS, Newcastle-Ottawa Scale.