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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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