Provides a “snapshot” of the frequency and characteristics of a disease in a population at a particular single point in time; basically identical to case-control study except that the variable assumed to be the cause of an event (or disease) is measured at the same time as the assignment of the patient to the event/disease category. In a cross-sectional survey, a specific group is looked at to → see if a substance or activity, say smoking, is related to the health effect being investigated - for example, lung cancer. If a significantly greater number of smokers already have lung cancer than those who don’t smoke, this would support the hypothesis that lung cancer is caused by smoking; c.s.s. usually measure prevalent outcomes, dropouts, fatal cases, migrants a.s.o. are not counted (example: assumption of a relationship between deep vein thrombosis and pills for birth control; if a true relationship exists the patient was taking the pill at the time when the thrombosis occurred; a history of pill-taking in the past would be much less conclusive); best suited for chronic, nonfatal conditions; disadvantages: frequently c.s.s. are unable to distinguish cause from effect, possibility for selection bias; → see also cohort study, case-control study.