Stepwise SPC Chart

The V-mask can now be placed over the graph with P-0 parallel to the x-axis and the point P over the current data point. If any part of the cumulative function protrudes the boundaries prescribed by the V-mask, we would conclude that the current mean of the process has deviated from the target. The CUSUM chart, in general, is more effective than the Shewhart Control Chart when used to monitor continuous processes that tend to drift over time. The CUSUM Chart, however, is quite vulnerable to the impact of process interruptions. Another drawback of the CUSUM Chart is that its direct relationship to the actual time variation of the process is not always clear, making it rather difficult for us to analyze and to improve the process. Refer to Stepwise SPC Chart. 

Chart employs exactly the same weighting function as the CUSUM Chart. To detect outside interruptions the Stepwise SPC Chart places 100% of the weight onto the last data point in the same way as the Shewhart Control Chart. The Stepwise SPC Chart is, therefore, a combination of the two common SPC charts the Shewhart Control Chart and the CUSUM Chart. Figure 5 illustrates the weighting function of the EWMA Chart. Most emphasis is placed on the last data point and regressively less emphases are placed on the previous data points. In this example, about 20.6% of the total weight is put on observation 16, (20.6 x 0.8)% of the total weight is put on observation 15 and (20.6 x 0.8 x 0.8)% is put on observation 14. 

This pattern, then, is a blend of the weighting functions employed by the Shewhart Control Chart and the CUSUM Chart. We make use of all data points yet more emphasis is placed on the recent ones. Although the Stepwise SPC Chart is a combination of the Shewhart Control Chart and the CUSUM Chart, the Exponentially Weighted Moving Average Chart is a compromise of the two. Figure 6 illustrates the EWMA. 

The Stepwise SPC Chart is a method that combines the functions of the CUSUM Chart and the Shewhart Control Chart. A sequential analysis is used to estimate the current mean of the process. Shewhart Control Charts are then constructed about the step functions representing the current mean. Figure 1 illustrates the Stepwise Control Chart. The step functions, based on sequential analysis, respond to process drift the same way as the CUSUM Chart. Each segment of the control chart then helps to identify the interruptions, in the same way as the Shewhart Control Chart does. With the proper choice of parameters, the Stepwise SPC Chart can effectively separate the three types of variations drift, fluctuations and interruptions, and thus help us to monitor and analyze the quality of the product. Refer to EWMA Chart. 

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