Table 6.
Checklist used for the assessment of the methodological quality of the included studies
| Theoretical background | Used for selection bias |
| 1. Is there a theoretical background for the hypothesis? | |
| Study participation | |
| 2. Is the study population clearly described in terms of age, gender, and important patients’ characteristics? | Used for selection bias |
| 3. Is the percentage of eligible subjects who participated in the study (response rate) adequate? (More than 30 % is considered inappropriate) | Used for selection bias |
| Sampling | |
| 4. Are patients who participated in the study similar to eligible non-participants (the population), in terms of age, gender, and important disease characteristics? | Used for selection bias |
| Study attrition | |
| 5. Is the percentage of subjects available for analysis adequate? (<30 % not too many missing values or loss to follow-up)? | Used for selection bias |
| 6. Were reasons for loss to follow-up presented and assessed during the study for possible systematic attrition? (Subjects that did not finish the study) | Used for selection bias |
| Outcome measurement | |
| Definition of outcome variable(s) | Used for information bias |
| 7. Are clear definitions of each outcome variable provided? | |
| 8. Is clear operationalization of each outcome variable provided? How is it measured? | Used for information bias |
| Measurement of outcome variable(s) | Used for information bias |
| 9. Are the measurement instruments used for the measurement of the outcome variable(s) reliable and valid? | |
| Method and setting of the outcome variable(s) | Used for information bias |
| 10. Were the measurement approach, time and place of measurement of the outcome variable(s) standardized or conducted in a way that limits systematically different measurement? | |
| Study confounding | |
| Definition of potential confounders | Used for information bias |
| 11. Are clear definitions of each confounder provided? | |
| 12. Is clear operationalization of each confounder provided? | Used for information bias |
| Measurement of potential confounders | Used for information bias |
| 13.Are the measurement instruments used for the measurement of the confounder(s) reliable and valid? | |
| Method and setting of the confounder(s) | Used for information bias |
| 14. Were the measurement approach, time and place of measurement of the confounder(s) standardized or conducted in a way that limits systematically different measurement? | |
| Statistical analyses | |
| 15. Is the percentage of missing values adequate? Less < 30 % | Used for statistical analysis bias |
| 16. Were multivariable analyses performed? Yes is rated as “0” if yes | Used for statistical analysis bias |
| 17. Was it clearly described which variables were included in the (multivariable) model(s)? | Used for statistical analysis bias |