Collaborative studies of comparative investigations of standardized drinking water were performed within Lower Saxony since two years. The results show that the precision of the measurements even in the reference laboratories is substantially less than the measuring tolerance given by the Federal Drinking Water Regulation. Therefore, the standard deviation of the different parameters, variation coefficients, respectively, are to be used for the internal statistical quality control as well as for the judgment of water. Protests against drinking water, bath water, respectively, due to parameters exceeded should be made just if the guide or imperative values are exceeded by the threefold standard deviation experimentally determined in reference laboratories.