Some of my colleagues have recently been pushing multivariate charts. These charts combine several different process outcome values into a single number that’s placed on a chart. Unfortunately, this theoretically elegant approach breaks down in practice for reasons that will be explained here.

We shall use the example of Adhesive 442 to illustrate the ideas behind multivariate charts. Each batch of Adhesive 442 is characterized by two key properties, pH and viscosity. Twenty consecutive batches had the values shown in Figure 1.

Figure 1: Adhesive 442 data

The average pH for Adhesive 442 is 7.9825. The average moving range for the pH values is 0.221. Based upon this value, the sigma(X) value for pH is 0.196. The XmR chart for the pH values is shown in Figure 2.

Figure 2: XmR chart for Adhesive 442 pH values

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