Study in which population is considered is the unit of study -

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