<prompt> In case control studiesMany patients are neededIncidence can be calculatedCause to effect is precededOdd ratio can be calculated  <response>: .case control studies are retrrspective studies,that proceeds from effect to cause.it requires two study groups,namely cases and controls.both exposure and outcome has occurred at the time of study.they are basically comparison studies,four basic steps of this incudes, selection of cases and controls matching measurement of exposure analysis and interpretation,from a case control study we can derive odd;s ratio that is a measure of association between risk factor and outcome.it is closely related to relative risk. ref:park;s textbook,ed22,pg no 70 