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***ALERT NOTE***
SAS Note: SN-002705
Product: QC
Component: SHEWHART
Title:
Tests may be incorrect for P and NP charts with TESTNMETHOD=STANDARDIZE
Keywords:
INCORRSTAT INCORROUT TESTS= TESTS FOR SPECIAL CAUSES PCHART NPCHART P NP
TESTNMETHOD=STANDARDIZE OUTTABLE= WRONG SIGNAL FLAG
In PROC SHEWHART, when the TESTNMETHOD=STANDARDIZE option is used on a
PCHART or NPCHART statement and the TESTS= option is specified, the
resulting positive test signals may be incorrect and positive tests may
not be flagged. This problem can occur when the computed upper control
limit for any subgroup exceeds 1 in P charts, or exceeds the subgroup
sample size in NP charts.
Specifically, the problem can occur in P charts when
_ _ _
p + 3*sqrt( p * (1 - p) / n ) > 1 for any subgroup i
i
and the problem can occur in NP charts when
_ _ _
(n * p ) + 3*sqrt( (n * p) * (1 - p) ) > n for any subgroup i
i i i
When this happens for the given chart type, subgroup i may be be
incorrectly included or omitted from contributing to a positive test
run. Both the printed plot and the OUTTABLE= data set will contain the
wrong results. There is no way to tell when false positive or false
negative signals have occurred as this will be data-dependent.
For P and NP charts, if the computed upper control limit does not exceed
1 or the subgroup sample size respectively for any subgroup, then all
tests flags will be correct.
There is no circumvention for this problem. The standardized values
could be computed in a DATA step and the tests manually applied. For P
charts, the standardized values for subgroup i are:
_ _ _
z = (p - p) / sqrt( p * (1 - p) / n )
i i i
For NP charts, the standardized values for subgroup i are:
_ _ _
z = (p - p) / sqrt( (n * p) * (1 - p) )
i i i
The zone boundaries are -3, -2, -1, 0, 1, 2, 3.
Keywords:
INCORRSTAT INCORROUT TESTS= TESTS FOR SPECIAL CAUSES PCHART NPCHART P NP
TESTNMETHOD=STANDARDIZE OUTTABLE= WRONG SIGNAL FLAG
Reported Releases:
+------------REPORTED------------+ +------FIXED-----+
System Version Level Version Level
AIX/R 7 B1 8 M0
CMS 7 B1 8 M0
DigitUnx 7 B1 8 M0
HP800 7 B1 8 M0
IABI 8 D1 8 M0
MIPS ABI 8 D1 8 M0
MVS 7 B1 8 M0
OS/2 7 B1 8 M0
Solaris 7 B1 8 M0
VMS 7 B1 8 M0
VMS/AXP 7 B1 8 M0
WIN/NT 7 B1 8 M0
Win95 7 B1 8 M0
Win98 7 P1 8 M0
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