<P> The case - control is a type of epidemiological observational study . An observational study is a study in which subjects are not randomized to the exposed or unexposed groups, rather the subjects are observed in order to determine both their exposure and their outcome status and the exposure status is thus not determined by the researcher . </P> <P> Porta's Dictionary of Epidemiology defines the case - control study as: an observational epidemiological study of persons with the disease (or another outcome variable) of interest and a suitable control group of persons without the disease (comparison group, reference group). The potential relationship of a suspected risk factor or an attribute to the disease is examined by comparing the diseased and nondiseased subjects with regard to how frequently the factor or attribute is present (or, if quantitative, the levels of the attribute) in each of the groups (diseased and nondiseased)." </P> <P> For example, in a study trying to show that people who smoke (the attribute) are more likely to be diagnosed with lung cancer (the outcome), the cases would be persons with lung cancer, the controls would be persons without lung cancer (not necessarily healthy), and some of each group would be smokers . If a larger proportion of the cases smoke than the controls, that suggests, but does not conclusively show, that the hypothesis is valid . </P> <P> The case - control study is frequently contrasted with cohort studies, wherein exposed and unexposed subjects are observed until they develop an outcome of interest . </P>

When is it desirable to use a case-control study (choose all that apply)