<Li> A randomized experiment may be impractical . Suppose a researcher wants to study the suspected link between a certain medication and a very rare group of symptoms arising as a side effect . Setting aside any ethical considerations, a randomized experiment would be impractical because of the rarity of the effect . There may not be a subject pool large enough for the symptoms to be observed in at least one treated subject . An observational study would typically start with a group of symptomatic subjects and work backwards to find those who were given the medication and later developed the symptoms . Thus a subset of the treated group was determined based on the presence of symptoms, instead of by random assignment . </Li> <Ul> <Li> Case - control study: study originally developed in epidemiology, in which two existing groups differing in outcome are identified and compared on the basis of some supposed causal attribute . </Li> <Li> Cross-sectional study: involves data collection from a population, or a representative subset, at one specific point in time . </Li> <Li> Longitudinal study: correlational research study that involves repeated observations of the same variables over long periods of time . </Li> <Li> Cohort study or Panel study: a particular form of longitudinal study where a group of patients is closely monitored over a span of time . </Li> </Ul> <Li> Case - control study: study originally developed in epidemiology, in which two existing groups differing in outcome are identified and compared on the basis of some supposed causal attribute . </Li> <Li> Cross-sectional study: involves data collection from a population, or a representative subset, at one specific point in time . </Li>

What type of data is collected in an observational study