Many erroneously think of a process behavior chart as a technique for monitoring a process. Consequently, they consider other monitoring techniques as alternatives to a process behavior chart. One of these proposed alternatives is an exponentially weighted moving average (EWMA). While an EWMA may be used to monitor a process and to make adjustments to the process aim, it is, at best, a poor echo of a process behavior chart.

Our example

The data in Figure 1 are the maximum measured diameters for a given camshaft bearing surface on 50 consecutively produced parts. The unit of measurement is 0.0001 in., and each value is the amount by which the diameter exceeds a reference value of 1.3700 in. The time order for these values is given by reading down each column, beginning with the column on the left.

Figure 1: Camshaft-bearing diameters, X

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