Observational studyCohort study · Epidemiological study

An observational study measures exposures and outcomes without assigning anyone to anything. Cohort studies follow groups forward in time; case-control studies look backwards from an outcome. Most of what is known about diet, sleep and activity at population scale comes from this design, because you cannot randomise people to thirty years of a lifestyle.

It matters because the design's strength and its weakness are the same fact: it studies real life. People who sleep well, eat more fibre or exercise more differ from those who don't in dozens of ways, only some of which can be measured and adjusted for. What remains is confounding, and it is why observational findings are usually reported as associations.

In practice, an observational finding is strengthened by a large effect size, a dose-response relationship, consistency across different populations, and a plausible mechanism — and it is genuinely settled only when a trial agrees, which for lifestyle questions is often never.

Sources

• Centre for Evidence-Based Medicine, University of Oxford: explanation of the 2011 OCEBM Levels of Evidence