SAS Institute Alert Note SN-005428

From: SAS Technical Support (LISTSERV@VM.SAS.COM)
Date: Fri Oct 12 2001 - 11:03:15 EDT

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    SN-005428 ***Alert Note***

    Interval censoring syntax may lead to incorrect results

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

      If there are any uncensored observations in the input dataset and you
      use interval censoring syntax (variable1 variable2) with the ANALYZE,
      PROBPLOT, or RELATIONPLOT statements, incorrect results will occur. The
      Turnbull algorithm for the computation of the empirical distribution
      function estimates may fail to iterate.

      With interval censoring syntax, uncensored observations are those for
      which variable1=variable2. Such observations represent exact failure
      times in the data.

      Results produced by the PPOUT and ITPRINTEM options of the ANALYZE,
      PROBPLOT, and RELATIONPLOT statements will be in error. Plots produced
      by PROBPLOT and RELATIONPLOT statements will also be incorrect. MODEL
      statements that do not use the Turnbull algorithm and are unaffected by
      this problem.

      To circumvent the problem, recode any uncensored observations as
      interval censored observations on a very small interval so that the
      intervals representing uncensored observations do not overlap the
      endpoints of any other interval. For uncensored observations
      (those where variable1=variable2), subtract a small positive value from
      variable1 and/or add a small positive value to variable2 keeping mind
      the proviso about overlapping endpoints. This workaround will produce
      correct probabilities and cumulative probabilities and affected plots
      will be correct. However, since the data are changed slightly results
      relating to the maximization of the log-likelihood such as parameter
      estimates will change. These changes are expected to be small.

    Keywords:
      procedure proc reliability qc sas/qc saspts incorrout incorrstat warning
      turnbull algorithm for the estimation of the cumulative distribution
      function did not converge in -1 iterations turnbull algorithm fails to
      iterate analyze probplot pplot relationplot rplot statement itpintem
      ppout option options incorrect results output cumulative probabilities
      interval censored censoring interval-censored interval-censoring syntax
      uncensored observations exact failure times (variable1 variable2)
      variable1 equals variable2 variable1=variable2 (lower upper) lower
      equals upper lower=upper incorrect plots graphs incorrect computation of
      plotting positions
      interval censoring syntax may lead to incorrect results
      005428 5428

    SAS Note Revised On: Thu, 11 Oct 2001

    System Version Reported Version Fixed

    AIX/6000 7 TSP1
    VM/ESA (CMS) 7 TSP1
    Compaq Tru64 UNIX 7 TSP1
    HP-UX Operating Systems 7 TSP1
    OS/390 (MVS) 7 TSP1
    IBM OS/2 7 TSP1
    Solaris 7 TSP1
    OpenVMS VAX 7 TSP1
    OpenVMS Alpha 7 TSP1
    Windows NT 7 TSP1
    Windows 95 7 TSP1
    Windows 98 7 TSP1

    No Fixes Available

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