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Tests for Special Causes |
The general syntax for a T-pattern is of the form
T( K=k M=m LOWER=a UPPER=b SCHEME=scheme CODE=character LABEL='label' LEGEND='legend' )
The options for a T-pattern are summarized in the following table:
Table 48.3: Options for T-PatternsOption | Description |
K=k | number of points |
M=m | number of consecutive points |
LOWER=value | lower limit of interval (a,b) |
UPPER=value | upper limit of interval (a,b) |
SCHEME=ONESIDED | one-sided scheme using (a,b) |
SCHEME=TWOSIDED | two-sided scheme using (a,b) (-b,-a) |
CODE=character | identifier for test (A-H) |
LABEL='label' | label for points that are signaled |
LEGEND='legend' | legend used with the TABLELEGEND option |
The following rules apply to the T-pattern options:
See SHWTSC4 in the SAS/QC Sample Library |
An example of a nonstandard test using a T-pattern is the run test based on 14 out of 17 points in a row on the same side of the central line that is suggested by Wheeler and Chambers (1986). The following statements apply this test with Tests 1, 3, and 4. The resulting chart is shown in Figure 48.11.
symbol v=dot c=salmon; title 'Analysis of Assembly Data'; proc shewhart history=assembly; xrchart offset * sample / mu0 = 20 sigma0 = 2.24 limitn = 5 alln tests = 1 t( k=14 m=17 lower=0 upper=. scheme=twosided code=A label='Test A' ) 3 4 vaxis = 16 to 26 by 2 split = '/' cinfill = ywh cframe = vligb cconnect = salmon ctests = black; label offsetx = 'Avg Offset in cm/Range'; run;
The specified T-pattern is signaled at 30 th subgroup. Consequently, this point is labeled Test A.
The general syntax for an M-pattern is of the form
M( K=k DIR=direction CODE=character LABEL='label' LEGEND='legend' )
The options for an M-pattern are summarized in the following table:
Table 48.4: Options for M-PatternsOption | Description |
K=k | number of points |
DIR=INC | increasing pattern |
DIR=DEC | decreasing pattern |
CODE=character | identifier for test (A-H) |
LABEL='label' | label for points that are signaled |
LEGEND='legend' | legend used with the TABLELEGEND option |
You must specify the direction of the pattern with the DIR= option. .
CAUTION: You should not substitute tests based on arbitrarily defined T-patterns and M-patterns for standard tests in general process control applications. The pattern options are intended primarily as a research tool.
See SHWARL2 in the SAS/QC Sample Library |
Champ and Woodall (1990) provide a FORTRAN program for assessing the run length distribution of tests based on T-patterns. A version of their algorithm is implemented by a SAS/IML program in the SAS/QC Sample Library.
If you specify either a T-pattern or M-pattern with the TESTS= option and save the results in an OUTTABLE= data set, the length of the variable _TESTS_ is 16 rather than 8 (the default). The ninth character of _TESTS_ is assigned the value A if the test with CODE=A is signaled, the tenth character of _TESTS_ is assigned the value B if the test with CODE=B is signaled, and so on. If you also specify one or more standard tests, the i th character of _TESTS_ is assigned the value i if Test i is signaled.
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