Table 3 Criteria for appraising risk for bias in descriptive studies
Type of risk | Low | Moderate | High |
|---|---|---|---|
Selection (sampling of population) | Consecutive unselected population Sample from general population not a selected group Rationale for case and control selection explained Follow-up or assessment time appropriate | Sample selected from large population, but selection criteria not defined Sample selection ambiguous, but may be representative Eligibility criteria not described Rationale for case and control selection not described Follow-up assessment not appropriate Analysis to adjust for sampling strategy bias | Sample selection ambiguous, and sample unlikely to be representative Highly selected population, making it difficult to generalise findings |
Selection (sampling size) if non-significant results; otherwise NA | Sample size calculation conducted and adequate | Sample size calculation not performed, but all eligible persons studied Sample size calculation performed, and reasons for not meeting sample size given | Sample size estimation unclear or only subsample studied |
Selection (participation rate) | High participation rate (> 85%) | Moderate participation rate (70-85%) | Low participation rate (> 70%) |
Performance bias (outcome assessment) | Diagnosis on basis of consistent criteria and direct examination | Assessment from hospital record or by questioning person Assessment from administrative database or register | Assessment from non-validated data or generic estimate from overall population |
Performance bias (confounding factors) | Control for common confounders (e.g. risk factors) | Only certain confounders adjusted (e.g. age) | Not controlled for any confounders |
Performance bias (blinding) when outcome and exposure relations being assessed (e.g. effect of GDM on offspring) | Outcome and exposure assessment independently blinded | Outcome and exposure assessment independently blinded | Â |
Performance bias (methods to control for bias) | Analysis appropriate for type of sample (e.g. subgroup analysis/regression etc.) | Analysis did not account for common adjustment | Data confusing |
Attrition bias | 0-10% attrition All participants from initiation to final outcome assessment accounted for Sensitivity analysis conducted for missing data | 11-20% attrition No sensitivity analysis for missing data | > 20% attrition and no sensitivity analysis for missing data |
Other: for comparative studies | Comparison groups similar at baseline | Baseline characteristics of comparison groups not assessed | Baseline characteristics dissimilar |