<prompt> In a study, group of smokers followed up for 10 years to find incidence of a cancer. What type of study design is this?Retrospective cohoProspective cohoCase control studyRandomized control trials  <response>: Types of coho study: 1. ProspectiveConcurrent Coho Begin in present and continue to future Outcome has not yet occurred at the time the investigation begins 2. RetrospectiveHistorical coho Outcomes have occurred before sta of investigation Investigator goes back in time to select study groups from existing records of past employment , up to the present 3. Ambidirectionalmixed coho Combination of retrospective and prospective coho studies 