SAS Institute Alert Note SN-008493

From: SAS Technical Support (
Date: Mon Oct 21 2002 - 08:10:22 EDT

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    SN-008493 ***Alert Note***

    Incorrect output is produced when using the THRESHOLD option in PROC

    Product: SAS/QC
    Component: RELIABILITY procedure
    Priority: ALERT

      The RELIABILITY procedure allows a fixed threshold, g, to be specified
      the Weibull, lognormal, and loglogistic distributions.

      The procedure computes estimates for the distribution parameters using Y
      = T - G as the response data, instead of the original lifetimes T. The
      parameter estimates for the distribution of Y are computed and reported
      correctly. However, some statistics that are reported on the scale of Y
      should be reported on the scale of T instead. Statistics describing the
      distribution location should be reported on the scale of the original
      data T, instead of the shifted data Y.

      Specifically, the following statistics are incorrectly reported.

      -Mean, Mode, Median, distribution percentiles and confidence limits

      MODEL statement:
      -Quantiles produced with the OBSTATS option

      The correct value for each of the statistics above is the reported value
      plus the threshold value G.

      In addition, survival function estimates created with the SURVTIME=
      options in the ANALYZE, PROBPLOT, and RELATIONPLOT statements assume a
      survival time on the scale of Y instead of T and will lead to incorrect
      results if the requested survival times are specified in terms of the
      original data T. Also, survival function estimates created with the
      OBSTATS(SURVIVAL) option on the MODEL statement do not account for the
      threshold parameter and are incorrect.

      Probability plots and relation plots created with the PROBPLOT and
      RELATIONPLOT statements must use Y as the lifetime variable and are

      incorrout incorrstat incorrect output produced threshold option proc
      reliability survival function estimates analyze probplot relationplot
      model statements obstats option survtime weibull lognormal loglogistic
      distribution sas/qc qc
      incorrect output is produced when using the threshold option in proc
      008493 8493

    SAS Note Revised On: Fri, 20 Sep 2002

    System Version Reported Version Fixed

    VM/ESA (CMS) 7 TSP1 9.1 TSD1
    Compaq Tru64 UNIX 7 TSP1 9.1 TSD1
    HP-UX Operating Systems 7 TSP1 9.1 TSD1
    OS/390 (MVS) 7 TSP1 9.1 TSD1
    IBM OS/2 7 TSP1 9.1 TSD1
    Solaris 7 TSP1 9.1 TSD1
    OpenVMS VAX 7 TSP1 9.1 TSD1
    OpenVMS Alpha 7 TSP1 9.1 TSD1
    Windows NT 7 TSP1 9.1 TSD1
    Windows 95 7 TSP1 9.1 TSD1
    Windows 98 7 TSP1 9.1 TSD1

    No Fixes Available

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