<prompt> Study in which population is considered is the unit of study -Cross sectionalEcologicalCase-controlCohort  <response>: Ans. is b i.e., Ecological Individuals as the unit of study - Case control study, Cohort study, Cross-sectional study. Population as the unit of study - Ecological correlational study. Ecological study is an epidemiological study in which the unit of analysis is a population rather than an individual. Therefore, ecological study utilizes data at the population level rather than the individual level. StudyUnit of study1.Observational studies A Descriptive studies formation of hypothesis i Case reportsii Case series B Analytic studies testing of hypothesis i Case - control or case - referenceii Cohort or follow-upiii Ecological or correlational studyiv Cross-sectional or pravalence studyIndividualsIndividualsPopulationIndividuals2.Experimental intervention studies testing of hvpothesis i Randamized controlled clinical trialsii Field trials or community intervention studiesiii Community trialsiv Animal studiesPatientsHealthy peopleCommunities 