<prompt> Observational study is all except -Case controlCohortRCTCross-sectional  <response>: Ans. is c i.e., RCT Ref: Park 22ndep. 60 2FVe p. 59; Modern epidemiology 3de p. 56Epidemiological studieso The primary concern of the epidemiologist is to study disease occurence in people,o The epidemiologist employs carefully designed research strategies to explore disease etiology,o Epidemiological studies may be of following types -StudyUnit of study1. Observational studiesA Descriptive studies formation of hypothesisi Case reportsii Case seriesB Analytic studies testing of hypothesisi Case - control or case - referenceii Cohort or follow-upiii Ecological or correlational studyiv Cross-sectional or prevalence study IndividualsIndividualsPopulationIndividuals2. Experimental intervention studies testing of hypothesisi Randamized controlled clinical trialsii Field trials or community intervention studiesiii Community trialsiv Animal studiesPatientsHealthy peopleCommunitieso In observational studies, the investigator measures but does not intervene and allow nature to take its own course - Epidemiologist studies the determinant of disease and its natural history. So that etiology of disease can be established and preventive control measure can be taken.Descriptive studies are used to formulate a hypothesisAnalytic studies are used for testing the hypothesiso In experimental studies, epidemiologist actively intervene to change a disease determinant or progression of disease.o Experimental studies are used for testing hypothesis. 