[The estimation of causal effect in observational studies]

One of the frequently encountered problems in the statistical analysis of the data is the correct interpretation of the effects occurring under the influence of some kind of treatment used by the researcher or appearing without its share of action. In the first case we are dealing with an experimental study, in the second with the observational study. In the experimental study, the researcher has full control over the procedure of random allocation of cases to a group subjected to a predetermined factor and a group not exposed to the action (randomization). In the case of an observational study, also called a retrospective study there are often additional factors that modify both assigned to the same groups as well as specific actions for the test results. The article presents an example of the cause of this phenomenon and selected methods of minimizing the mentioned influence.