Re: [sc] safety language Was:Campaign for Plain English



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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.)


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