CCHART Statement: SHEWHART Procedure

Example 17.8 Applying Tests for Special Causes

Note: See Tests for Special Causes Applied to c Chart in the SAS/QC Sample Library.

This example illustrates how you can apply tests for special causes to make c charts more sensitive to special causes of variation. Twenty trucks of the same model are inspected, and the number of paint defects per truck is recorded. The following statements create a SAS data set named Trucks3: 

data Trucks3;
   input TruckID $ Defects @@;
   label TruckID='Truck Identification Number'
         Defects='Number of Paint Defects';
   datalines;
B1  12    B2  4    B3  4    B4   3
B5   4    D1  2    D2  3    D3   3
D4   2    D9  4    M2  9    M6  13
L3   5    L4  4    L7  6    Z1  15
Z2   8    Z3  9    Z7  6    Z9   8
;

The following statements create a c chart and tabulate the information on the chart. The chart and table are shown in Output 17.8.1 and Output 17.8.2. 

ods graphics on;
title1 'c Chart for Paint Defects in New Trucks';
title2 'Tests=1 to 4';
proc shewhart data=Trucks3;
   cchart Defects*TruckID / tests      = 1 to 4
                            testlabel1 = 'Test=1 Signaled'
                            testlabel2 = 'Test=2 signaled'
                            odstitle   = title
                            odstitle2  = title2
                            zonelabels
                            tabletests
                            tablelegend;
run;

The TESTS= option requests Tests 1, 2, 3, and 4, which are described in Tests for Special Causes: SHEWHART Procedure. Only Tests 1, 2, 3, and 4 are recommended for c charts. The TESTLABEL1= and TESTLABEL2= options specify the labels for points where Tests 1 and 2 are positive. The TESTFONT= option specifies the font for the labels indicating points at which the tests are positive.

Output 17.8.1: Tests for Special Causes Displayed on c Chart

Tests for Special Causes Displayed on Chart


Output 17.8.2: Tabular Form of c Chart

c Chart for Paint Defects in New Trucks
Tests=1 to 4

The SHEWHART Procedure

c Chart Summary for Defects
TruckID Subgroup
Sample
Size 		3 Sigma Limits with n=1 for
Count 		Special
Tests
Signaled
Lower
Limit 		Subgroup
Count 		Upper
Limit
B1 1.00000 0 12.000000 13.669940  
B2 1.00000 0 4.000000 13.669940  
B3 1.00000 0 4.000000 13.669940  
B4 1.00000 0 3.000000 13.669940  
B5 1.00000 0 4.000000 13.669940  
D1 1.00000 0 2.000000 13.669940  
D2 1.00000 0 3.000000 13.669940  
D3 1.00000 0 3.000000 13.669940  
D4 1.00000 0 2.000000 13.669940  
D9 1.00000 0 4.000000 13.669940 2
M2 1.00000 0 9.000000 13.669940  
M6 1.00000 0 13.000000 13.669940  
L3 1.00000 0 5.000000 13.669940  
L4 1.00000 0 4.000000 13.669940  
L7 1.00000 0 6.000000 13.669940  
Z1 1.00000 0 15.000000 13.669940 1
Z2 1.00000 0 8.000000 13.669940  
Z3 1.00000 0 9.000000 13.669940  
Z7 1.00000 0 6.000000 13.669940  
Z9 1.00000 0 8.000000 13.669940  

Test Descriptions
Test 1 One point beyond Zone A (outside control limits)
Test 2 Nine points in a row on one side of center line 




            

            


            
The ZONELABELS option requests zone lines and displays zone labels on the chart. The zones are used to define the tests. The
               TABLETESTS option requests a table of counts of nonconformities, subgroup sample sizes, and control limits, together with
               a column indicating the subgroups at which the tests are positive. The TABLELEGEND option adds a legend describing the tests
               that are positive. 
            

            
Output 17.8.1 and Output 17.8.2 indicate that Test 1 is positive at Truck Z1 and Test 2 is positive at Truck D9.