A research design in which investigators observe and record outcomes without assigning treatments or interventions. Participants are not randomized; the researcher studies existing groups or follows individuals making their own treatment decisions.

Observational studies include cohort studies (following groups forward in time), case-control studies (comparing people with and without an outcome), and cross-sectional studies (measuring everything at one time point). They can identify associations and generate hypotheses but cannot establish causation—because participants self-select into treatment groups, differences in outcomes may reflect pre-existing differences between groups rather than treatment effects. In the peptide evidence hierarchy, observational data ranks below randomized controlled trials but above case reports and community anecdotes.