Case–control study versus<unk>  on a timeline. "OR" stands for "odds ratio" and "RR" stands for "relative risk". A<unk>  is a longitudinal<unk>  study that follows over time a group of similar individuals<unk> s) who differ with respect to certain factors under study, to determine how these factors affect rates of a certain outcome. For example, one might follow a<unk>  of middle-aged truck drivers who vary in terms of smoking habits, to test the hypothesis that the 20-year incidence rate of lung cancer will be highest among heavy smokers, followed by moderate smokers, and then nonsmokers. The<unk>  study is important for research on the etiology of diseases and disorders. The distinguishing feature of a<unk>  is that at the time that the investigators begin enrolling subjects and collecting baseline exposure information, none of the subjects have developed any of the outcomes of interest. After baseline information is collected, subjects in a<unk>  are then followed "longitudinally," i.e. over a period of time, usually for years, to determine if and when they become diseased and whether their exposure status changes outcomes. In this way, investigators can eventually use the data to answer many questions about the associations between "risk factors" and disease outcomes. For example, one could identify smokers and non-smokers at baseline and compare their subsequent incidence of developing heart disease. Alternatively, one could group subjects based on their body mass index (BMI) and compare their risk of developing heart disease or cancer.<unk> es are typically ranked higher in the hierarchy of evidence than retrospective<unk>  studies and can be more expensive than a case–control study. One of the advantages of<unk> es is they can help determine risk factors for being infected with a new disease because they are a longitudinal observation over time, and the collection of results is at regular time intervals, so recall error is minimized.
Prospective cohort study
Natural history study
Retrospective cohort study
Placebo-controlled study
Randomized controlled trial
Cohort study
Nested case–control study
Case–control study