<prompt> Incidence is calculated by-Retrospective studyProspective studyCross sectional studyRandom study  <response>: Ans. is b i.e., Prospective study o Incidence rate is measured by cohort study, also known as prospective study.Epidemiological studyParameter obtainedo Case control studyOdds ratioo Cohort studyIncidence rateRelative riskAttributable riskPopulation attributable risko Cross - sectional studyPrevalenceo Ecological studyGroup characteristics 