Shewhart, CUSUM and EWMA Control Charts: A Comparative Study on Intermediate Check of Balances

The quality assurance of the measurement results generated by ISO/IEC 17025 testing and calibration laboratories is directly associated with monitoring the processes performed by their measuring instruments. This monitoring can be performed through the application of control charts, and, for this study, four electronic balances belonging to two different laboratories were chosen as the measuring instruments analyzed. The main goal of this research is to identify, among the cumulative summation (CUSUM) control charts, exponentially weighted moving average (EWMA) and Shewhart control charts, the most adequate to monitor the trends of these balances. It was necessary to define the periods of analysis of the data collected by the laboratories, to calculate the control limits of each chart (phase I), to carry out the monitoring of the process (phase II), followed by the comparison between the three types of control charts. The Minitab Statistical 17 software was used for the construction of these control charts. This study identified that the EWMA control chart is the best alternative to monitor trends at all points of the mid-check since it is the only one that can be used with both normal and non-normal data. However, it was not possible to identify which control chart detects the special causes earlier.