Confirmation Sample Control Charts

Control charts such as X and R charts are widely used in industry to monitor quality. These charts are effective in detecting large departures from the in-control condition. Other process monitoring methods, such as Cumulative Sum (CUSUM) charts are sequential in nature as they accumulate information from previous observations. CUSUM charts are good at detecting more moderate presistent process shifts, but are more complicated to implement. As a compromise, a number of researchers have investigated adapting the simple Shewhart type control charts (X and R) to take into account some but not all the previous observations. Variable Sampling Interval (VSI) control charts adjust the sampling interval based on the current observation. If there is some evidence (but no out-of-control signal) that the process may have shifted the sampling interval is shortened. This has the effect of reducing the number of non-conforming parts produced if there moderate process shift. VSI X charts are discussed by Reynolds, Amin, Arnold, Nachlas 1988. The VSI approach is most effect for sequential charts such as CUSUM charts since sample statistics plotting close to the control limit have accumulated significant evidence against the in-the sample size in response to the observed statistics. Variable sample size (VSS) control charts suggest X charts that occasionally require a second sample. These two approaches suggest adding warning limits to the standard control