LISA 2001 Abstract
A Probabilistic Approach to Estimating
Computer System Reliability
Robert Apthorpe Excite@Home, Inc.
Probabilistic Risk Assessment (PRA) is a method of estimating
system reliability by combining logic models of the ways systems can
fail with numerical failure rates. One postulates a failure state and
systematically decomposes this state into a combination of more basic
events through a process known as Fault Tree Analysis (FTA). Failure
rates are derived from vendor specifications, historical trends, on-
call reports, and many other sources. FTA has been used for decades in
the defense, aerospace, and nuclear power industries to manage risk
and increase reliability of complex engineering systems. Combining FTA
with event tree analysis (ETA), one can associate failure
probabilities with consequences to clearly communicate risk both
pictorially and numerically. Basic PRA techniques can help increase
the reliability and security of computer systems.
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