In the hierarchy of evidence, which type typically provides the least rigorous evidence?

• A. Case reports / case series
• B. Cohort studies
• C. Randomized controlled trials
• D. Expert opinion

Answer: (D)

In evidence-based practice, sources are ranked by how rigorously they test hypotheses and control bias. Expert opinion sits at the bottom because it relies on individual judgment without systematic data, formal study design, or reproducibility. Case reports and case series describe single or few patients and can signal patterns, but without comparison groups they can’t determine cause and effect or generalize widely. Cohort studies follow groups over time and can show associations while controlling for some confounders, making them more reliable than purely descriptive reports. Randomized controlled trials randomly assign participants to intervention or control, with blinding and predefined outcomes, minimizing bias and enabling stronger causal inferences. Therefore, expert opinion provides the least rigorous evidence in the hierarchy. Use it only when higher-quality data aren’t available, and seek stronger evidence to guide practice.