Biostatistics, Epidemiology & EBM

Randomization of subjects to intervention vs control. Random allocation distributes both known and unknown confounders equally between groups, so a difference in outcome can be attributed to the intervention itself. RCTs are prospective and interventional.

A cohort study starts with EXPOSURE status (exposed vs unexposed) and follows subjects forward to see who develops the OUTCOME. A case-control study starts with OUTCOME status (cases with disease vs controls without) and looks BACKWARD to compare exposures.

The odds ratio (OR). A case-control study starts by selecting on disease status, so the investigator fixes the number of cases and controls and cannot measure incidence/risk in the population — therefore relative risk cannot be computed directly. The OR approximates RR when the disease is rare.

Relative risk (RR), because a cohort study follows exposed and unexposed groups forward and can measure the incidence (risk) of the outcome in each group. RR = incidence in exposed / incidence in unexposed.

A cross-sectional study measures exposure and outcome SIMULTANEOUSLY at a single point in time (a 'snapshot'), so it estimates PREVALENCE and associations. Its key limitation: because exposure and disease are assessed at once, it usually cannot establish temporality (which came first), so it cannot establish causation.

A case series is a descriptive report of a group of patients with a similar diagnosis or exposure, with no comparison/control group. It is near the bottom of the evidence hierarchy (above only case reports and expert opinion); it can generate hypotheses but cannot test associations or causation.