TOPICS: Cross-sectional, cohort, case-control, randomized-controlled trial (RCT), crossover study, case series, ecological, prevalence, incidence, correlation, causation, exposure, control, case, risk factor, relative risk (RR), retrospective, prospective, confounding, odds ratio, treatment, placebo, single-blinding, double-blinding, random assortment, washout period, population, individual, group factors, ecological fallacy
