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Figure 6.3. A dot diagram of individual observations of a variable.

standard deviation chart (S chart) monitors within sample process variation or spread (process variability at a given time). The x chart must be used along with a spread chart. The process spread must be in-control for proper interpretation of the x chart.

Usually several observations of the same variable at a specific time are used (Figure 6.3). If only one observation is available, individual values can

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