Table 2.

Criteria for evaluating the internal validity of individual studies according to the United States Preventive Services Task Force [21].^a

Study design	Criteria
Systematic reviews	▪ Comprehensiveness of sources and search strategy used ▪ Standard appraisal of included studies ▪ Validity of conclusions ▪ Recentness and relevance
Case–control studies	▪ Accurate ascertainment of cases ▪ Non-biased selection of cases and controls with exclusion criteria applied equally to both ▪ Response rate ▪ Diagnostic testing procedures applied equally to each group ▪ Appropriate attention to potential confounding variables
RCTs and cohort studies	▪ For RCTs: adequate randomization, including concealment and whether potential confounders were distributed equally among groups ▪ For cohort studies: consideration of potential confounders with either restriction or measurement for adjustment in the analysis; consideration of inception cohorts ▪ Maintenance of comparable groups (includes attrition, crossovers, adherence, or contamination) ▪ Important differential loss to follow-up or overall high loss to follow-up ▪ Measurements: equal, reliable, and valid (includes masking of outcome assessment) ▪ Clear definition of interventions ▪ All important outcomes considered ▪ Analysis: adjustment for potential confounders for cohort studies, or intention-to-treat analysis for RCTs
Diagnostic accuracy studies	▪ Screening test relevant, available for primary care, adequately described ▪ Study uses a credible reference standard, performed irrespective of test results ▪ Reference standard interpreted independently of screening test ▪ Handles indeterminate results in a reasonable manner ▪ Range of patients included in study ▪ Sample size ▪ Administration of reliable screening test

Abbreviation: RCT, randomized controlled trial.

^aReproduced from Harris et al. [21], by permission of Elsevier.