Hygienic and sanitary research, as a rule, are related to the selective control. In this case, the decision on the quantitative characteristics of environmental factors or public health is based on the results of checking one or more selections of samples, and environmental quality is assessed by comparing the values of these characteristics with standards, primarily – maximum allowable concentrations of pollutants. Strict legal regulation of such work is necessary, but the current regulatory documents are incomplete and even contradictory. There are erroneous decisions due to unrepresentativeness, not only to the insufficient amount of initial data, but also to their unsatisfactory processing. The consequence of such decisions is unjustified requirements for enterprises that pollute the environment, charging inflated payments for a negative impact on the environment, etc. Using the example of monitoring for highly variable indicators of the quality of natural and waste waters, showed that in order to make a correct conclusion about the fulfillment or violation of hygienic and sanitary requirements, based on a comparison of several samples, it is necessary to equalize their statistical weights. Violation of this condition can lead to the dependence, oriented in one direction in stratified datasets, will acquire the opposite direction after their summation. In preventing such errors, the role of territorial bodies of sanitary and epidemiological control and supervision is essential.

Compliance with ethical standards: the article does not deal with issues regulated by ethical standards.