<prompt> Epidemiological studies is carried for a period of -Incubation periodTwice the incubation periodFour times the incubation periodHalf the incubation period  <response>: .epidemiological studies are usually employed for a period of twice the incubation period .it is done so so thzt most of the cases and associated risk factors are identified in such a way that it may help in the interpretation of data collected and thus the epidemiological analysis. ref:park;s textbook,ed 22,pg no 67 