Note: See Nonnormal Process Data in the SAS/QC Sample Library.

A number of authors have pointed out that Shewhart charts for subgroup means work well whether the measurements are normally distributed or not.[55] On the other hand, the interpretation of standard control charts for individual measurements (X charts) is affected by departures from normality.

In situations involving a large number of measurements, it may be possible to subgroup the data and construct an chart instead of an X chart. However, the measurements should not be subgrouped arbitrarily for this purpose.[56] If subgrouping is not possible, two alternatives are to transform the data to normality (preferably with a simple transformation such as the log transformation) or modify the usual limits based on a suitable model for the data distribution.

The second of these alternatives is illustrated here with data from a study conducted by a service center. The time taken by staff members to answer the phone was measured, and the delays were saved as values of a variable named Time in a SAS data set named Calls. A partial listing of Calls is shown in Figure 17.214.