Table 2.

Risk of Bias Assessment

Study	Study participation	Study attrition	Prognostic factor measurement	Outcome measurement	Study confounding	Statistical analysis and reporting	Overall
Tapper et al.	Moderate	Low	Moderate	Moderate	Moderate	Low	Moderate
Stineman et al.	High	Low	Moderate	Moderate	Moderate	Low	High
Fairchild et al.	High	High	Moderate	Moderate	Moderate	Moderate	High
Simonet et al.	Moderate	Moderate	Low	Low	Low	Low	Moderate
Basic et al.	Low	Low	Moderate	Low	Moderate	Moderate	Moderate
Mehtah et al.	High	Low	Low	Low	Moderate	Low	Moderate

For each category low risk of bias is defined as: Study participation: The study sample represents the population of interest on key characteristics, Study Attrition: Loss to follow-up is not associated with key characteristics, Prognostics Factor Measurement: Factors are adequately measured, Outcomes Measurement: Outcome of interest is adequately measured, Study Confounding: Important potential confounders are appropriately accounted for, Statistical Analysis and Confounding: The statistical analysis is appropriate for the design of the study, Overall: Majority of criteria met. Little or no risk of bias.