SAS Note 002705

From: tsdlist@unx.sas.com
Date: Mon Aug 21 2000 - 13:44:14 EDT

  • Next message: tsdlist@unx.sas.com: "SAS Note V6-CLUSTER-G773"

    >>>>>>>> SAS Notes -- Copyright (c) 1991 - 2000 by SAS Institute Inc. <<<<<<<<

                                      ***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



    This archive was generated by hypermail 2b29 : Mon Aug 21 2000 - 13:44:47 EDT