SINGLE VERSUS DUAL PROCESS CONTROL CHARTS

Statistical process control (SPC) charts have become a standard tool in manufacturing quality control. Measurements of a product characteristic are plotted on a time chart. The chart is marked with an upper control limit (UCL) and a lower control limit (LCL). The UCL and LCL are based on a specified equal number of standards deviations above and below the mean value of the measurement. The manufacturing process used to make the product is deemed to be in control if all measurements fall within the UCL and LCL. The process is deemed to be out of control if a measurement falls above the UCL or below the LCL. An out of control process is stopped, the cause of the out of control condition is identified and rectified, and the process restarted. The statistical design of the traditional chart is based on the assumption that the measurements are independent of each other, and normally and identically distributed (NID). However, we now know from Alwan and Roberts, 1995, that in about 80 percent of cases, actual process measurements are not NID. There are serially correlated.