From: C. Michael Holloway (c.m.holloway(at)larc.nasa.gov)
Date: Wed 04 Dec 2002 - 15:45:24 GMT
>Trying to remedy Alan's point about this discussion being (a posteriori >:-) more for linguists than safety engineers, I would be interested if any >of the practitioners out there have come across any thorny - and >specifically safety-related - terminological issues (say when agreeing or >applying standards, or wording public statements). There is considerable debate within the commercial aviation community about the meaning of certain terms relevant to Modified Condition / Decision Coverage (MC/DC) within DO-178B/ED-12B. This debates stems in large part from the terms used in the document to define (using that word quite loosely) MC/DC. The relevant glossary entries are as follows: "Condition–A Boolean expression containing no Boolean operators." "Decision–A Boolean expression composed of conditions and zero or more Boolean operators. A decision without a Boolean operator is a condition. If a condition appears more than once in a decision, each occurrence is a distinct condition." "Modified Condition/Decision Coverage–Every point of entry and exit in the program has been invoked at least once, every condition in a decision in the program has taken all possible outcomes at least once, every decision in the program has taken all possible outcomes at least once, and each condition in a decision has been shown to independently affect that decision’s outcome. A condition is shown to independently affect a decision’s outcome by varying just that condition while holding fixed all other possible conditions." To see the difficulties caused by this terminology, try to use the above entries to determine, how many 'conditions' are in the following expression: (A and B) or (B and C) or (A and C) Typical answers given, even by commercial aviation software experts, include 3, 4, 6, & 9. For a more detailed discussion of these issues see section 2.6 of A Practical Tutorial on Modified Condition/Decision Coverage, NASA/TM-2001-210876, May 2001, by Kelly J. Hayhurst, Dan S. Veerhusen, John J. Chilenski and Leanna K. Rierson. PDF and Postscript versions are available at http://techreports.larc.nasa.gov/ltrs/refer/2001/tm/NASA-2001-tm210876.refer.html - cMh --- C. Michael Holloway, Senior Research Engineer NASA Langley Research Center, Hampton Virginia (As everyone almost certainly knows, the opinions expressed herein are mine, and do not in any way represent official opinions of NASA Langley.) [The content of this part has been removed by the mailing list software]