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Specialized Control Charts |
See SHWNONN 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.* 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.* 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 listing of CALLS is shown in Figure 49.24.
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