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Figure 6.6. The CUSUM status chart.

The disadvantages of the MA charts are slow response to sudden shifts in level and the generation of autocorrelation in computed values.

Estimation of S for individual measurements

Three approaches can be used for estimating S:

1. If a rational blocking of data exists, compute an estimate of S based on it. It is advisable to compare this estimate with the estimates obtained by using the other methods to check for discrepancies.

2. The overall S estimate. Use all the data together to calculate an overall standard deviation. This estimate of S will be inflated by the between-sample variation. Thus, it is an upper bound for S. If there are changes in process level, compute S for each segment separately, then combine them by using

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