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UCHART Statement |
See SHWUEX1 in the SAS/QC Sample Library |
This example illustrates how you can apply tests for special causes to make u charts more sensitive to special causes of variation.
A textile company inspects rolls of fabric for defects. The rolls are one meter wide and 30 meters long. The following statements create a SAS data set named FABRIC3, which contains the number of fabric defects for 20 rolls of fabric:
data fabric3; input roll defects @@; datalines; 1 6 2 9 3 14 4 17 5 3 6 8 7 9 8 2 9 14 10 1 11 3 12 5 13 6 14 9 15 10 16 12 17 11 18 4 19 9 20 4 ;
The following statements create a u chart and tabulate the
information on the chart.
The chart and tables are shown in Output 41.1.1 and Output 41.1.2.
title1 'u Chart for Fabric Defects'; title2 'Tests=1 to 4'; symbol v=dot c=vig; proc shewhart data=fabric3; uchart defects*roll / subgroupn = 30 tests = 1 to 4 ltests = 20 zonelabels tabletests cframe = steel cinfill = ligr ctests = black coutfill = yellow cconnect = vig; run;
The TESTS= option requests Tests 1, 2, 3, and 4, which are described in Chapter 48, "Tests for Special Causes." Only Tests 1, 2, 3, and 4 are recommended for u charts. The ZONELABELS option requests the zone lines, which are used to define the tests, and displays labels for the zones. The LTESTS= option specifies the line type used to connect the points in a pattern for a test that is signaled. The TABLETESTS option requests a table of the values of ui and the control limits, together with a column indicating the subgroups at which the tests are positive.
Output 41.1.1 and Output 41.1.2 indicate that Test 1 is positive for Roll 4 and Test 3 is positive at Roll 15.
Output 41.1.1: Tests for Special Causes Displayed on u ChartOutput 41.1.2: Tabular Form of u Chart
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