<prompt> Incidence of diarrhoea in a community can be calculated by-Case control studyCoho studyCross sectional studyDouble blind placebo control study  <response>: Coho study is also known as incidence study. The cohos are identified prior to the appearance of disease under investigation. The study group so defined are observed over a period of time to determine the frequency of disease among them. refer pgno:75 park 23rd edition 