Observational study is the study in which the objective is to discover cause-and-effect relationships but in which it is not feasible to use the controlled experimentation, in the sense of being able to impose process or treatments whose effects it is desired to discover, or to allocate the subjects at random to different measures. Surveys and most epidemiological studies come into this class. As the investigator does not control assignment of treatments there is no means to ensure that similar subjects receive different treatments. The classical instance of such a study that the successfully uncovered evidence of the important causal relationship is the smoking and lung cancer investigation of Doll and Hill.