<prompt> Group of smokers followed up for 10 years to find incidence of cancer lung. What type of study is this?Retrospective cohortProspective cohortCase control studyRandomized control trials  <response>: B i.e. Prospective cohort studyRef: Park, 24th edition, page 80; Leon Gordis, Epidemiology, 5th editionExplanation:Cohort study is otherwise called as incidence study. Cross sectional study is otherwise called as prevalence study.Comparison of case control and cohort studiesS.No.FeaturesCase control studiesCohort studies1Study groupPersons with disease outcomeExposed persons risk factor2Comparison groupPersons without disease controlsNon exposed persons3Measurements of riskOdds ratioRelative risk, attributable risk, Population attributable risk4Natural history of diseaseHard to establishEasily establish5Time of studyShortLong6CostInexpensiveExpensive7Sample size neededSmallLarge8Best whenDisease is rareExposure is rare9BiasSelection bias occur during selection of cases and controlsHawthorne bias 