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Introduction |
Figure 1 illustrates a typical Shewhart chart.
All Shewhart charts have the following characteristics:
The control limits are also determined by the subgroup sample size because the standard error of the summary statistic is a function of sample size. If the sample size is constant across subgroups, the control limits are typically horizontal lines, as in Figure 1. However, if the sample size varies from subgroup to subgroup, the limits are usually adjusted to compensate for the effect of sample size, resulting in step-like boundaries.
Control limits can be estimated from the data being analyzed, or they can be standard, previously determined values. Estimated limits are often used when statistical control is being established, and standard limits are often used when statistical control is being maintained.
When the process is in statistical control, a point may fall outside the control limits purely by chance, resulting in a false out-of-control signal. However, when the Shewhart chart correctly signals the presence of a special cause, additional action is needed to determine the nature of the problem and eliminate it.
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