1 2 3 4 5 6 7 8 9 1011 1213141516171819202122232425 Observation Number

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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