Such a study often is called a retrospective study since the study collects data by looking back in time. An observational study of the linkage between smoking and lung cancer can be a useful source of data but must be interpreted carefully since many factors that are difficult to assess may contribute to the data being collected and resultant conclusions. In the example although studies have suggested a strong connection between cigarette smoking and lung cancer, only recently has the mechanism by which cigarette smoking can cause lung cancer been identified and understood. A different approach is needed when an intervention such as a new medicine, treatment, process or change has been developed and one wants to measure its impact or effectiveness. Then some form of experiment is needed that can assess the impact of the intervention and separate that impact from a large number of other factors.
Several approaches are possible.
The medicine could be administered to a group of volunteers and its effect determined by follow-up examinations and interviews. Such an investigation is called an uncontrolled evaluation or experiment and is used widely, particularly when the context of the medicine, change or environment being evaluated does not allow a more controlled evaluation. However, an uncontrolled experiment has several limitations. For example, are any improvements identified during the test of the drug company's new medicine due to the medicine or due to some other factor such as a change in the diet or exercise regime of the individuals receiving the drug. Another limitation in an uncontrolled experiment is that the subjects used in the study may not be representative of the population of interest.