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D. Tang, H. Hecht, M. Hecht, "Software Dependability Assessment - Myth and
Reality", Issues in NASA Program and Project Management No 12, Washington, DC, June
1997
The importance of software as a contributor (if not the actual cause) of catastrophic
events has been well documented [Leveson95]. Moreover, as software is integrated
into safety critical systems, the same quantitative reliability requirements which
have been previously allocated to hardware are now being allocated to both hardware
and software. For example, both U.S. Federal Aviation Regulations (FARs) and International
Joint Aviation Regulations impose maximum acceptable probabilities for failures
of systems in passenger transport aircraft. Part 10 of the U.S. Code of Federal
Regulations also establishes maximum acceptable probabilities for radioactive releases
from nuclear power plants. When these standards were written, analog control systems
were the dominant technology, and there was an accepted methodology for reliability
prediction. Now digital (i.e., software-based) systems are replacing analog controls,
but the old standards remain in force. The need for updating the standards and methodology
extends to unregulated fields (e.g., computer-based automobile electronics) where
there is economic motivation to being able to quantify the expected failure behavior.
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