This improves the efficiency of comparison and also the validity of the study by avoiding errors.

Cases are made identical with controls i.e. all the known variables of the cases like age, sex, occupation, social status etc are neutralized with those of controls except the factor (disease) under study.

For example, cases and controls of same age, same sex, same occupation and living same village may be selected for better comparison, if feasible.

Like should be compared with the like principle is to be followed. When every thing remains common and same for both the groups and if the cause is found excessive in the cases than in controls, then it is presumed to cause the disease in the cases.