Elements of Box-and-Whisker Plots

A box-and-whisker plot is displayed for the measurements in each group on the ANOM boxchart. Figure 4.9 illustrates the elements of each plot.

Figure 4.9 Box-and-Whisker Plot

The skeletal style of the box-and-whisker plot shown in Figure 4.9 is the default. You can specify alternative styles with the BOXSTYLE= option; see the entry for the BOXSTYLE= option in Dictionary of Options.

Central Line

By default, the central line on an ANOM chart for means represents the weighted average of the group means, which is computed as

You can specify a value for with the MEAN= option in the BOXCHART statement or with the variable _MEAN_ in a LIMITS= data set.

Decision Limits

In the analysis of means for continuous data, it is assumed that the responses in the th group are at least approximately normally distributed with a constant variance:

When the group sizes are constant (), then and the decision limits are computed as follows:

Here the mean square error (MSE) is computed as follows:

For details concerning the function , see Nelson (1981, 1982a, 1993).

When the group sizes are not constant (the unbalanced case), and the decision limits for the th group are computed as follows:

Here the mean square error (MSE) is computed as follows:

This requires that be positive. A chart is not produced if but MSE is equal to zero (unless you specify the ZEROSTD option). For details concerning the function , see Fritzsch and Hsu (1997), Nelson (1982b, 1991), and Soong and Hsu (1997).

You can specify parameters for the limits as follows:

• Specify with the ALPHA= option or with the variable _ALPHA_ in a LIMITS= data set. By default, = 0.05.

• Specify a constant nominal sample size for the decision limits in the balanced case with the LIMITN= option or with the variable _LIMITN_ in a LIMITS= data set. By default, is the observed sample size in the balanced case.

• Specify with the LIMITK= option or with the variable _LIMITK_ in a LIMITS= data set. By default, is the number of groups.

• Specify with the MEAN= option or with the variable _MEAN_ in a LIMITS= data set. By default, is the weighted average of the responses.

• Specify with the MSE= option or with the variable _MSE_ in a LIMITS= data set. By default, is computed as indicated above.

• Specify with the DFE= option or with the variable _DFE_ in a LIMITS= data set. By default, is determined as indicated above.