Control charts for the toxicity of finished water—Modeling the structure of toxicity

The aim of this study was to construct control charts for the data that were obtained from the daily toxicological analysis of finished water (that is the water which has already been treated for human consumption) from the treated-water tanks of the Water Supply & Sewerage Corporation of Athens. The basic idea of the control charts is to test the hypothesis that there are only common causes of variability versus the alternative that there are special causes. The control charts are designed and evaluated under the assumption that the observations from the process are independent and identically distributed (iid) normally. However, the independence assumption is often violated in practice. Time or serial dependence (autocorrelation) may be present in many chemical procedures, and may have a significant effect on the properties of the control charts. The fact that a serious amount of autocorrelation was present in the data for the toxicity of the fished water had a serious impact on the typical control charts. The problem of the autocorrelation was overcome by the usage of a more sophisticated method based on time-series ARIMA models.