OLe Safety vs. Reliab ility Does Software Fail? Failure: Nonperformance or inability of system or component to perform its intended function for a specified time under specified environmental conditions A basic abnormal occurrence, e.g burned out bearing in a pump relay not closing properly when voltage applied Fault: Higher-order events, e.g relay closes at wrong time due to improper functioning of an upstream component All failures are faults but not all faults are failures Reliability Engineering Approach to Safety Reliability: The probability an item will perform its required function in the specified manner over a given time eriod and under specified or assumed conditions (Note: Most software-related accidents result from errors in specified requirements or function and deviations from assumed conditions. Concerned primarily with failures and failure rate reduction Parallel redundancy Standby sparing Safety factors and margins Derating Screening Timed replacementsc ✌☎✄☎✍✎✄☎✏✎✠☎✑✝✒✔✓✭★ ✮✛☎✯✄☎✚✰✜✍✎✏✲✱☎✣✥✄☎☛ ✤✛☎✡☎✤ ☛ ✤✚✰ Does Software Fail? Failure: Nonperformance or inability of system or component to perform its intended function for a specified time under specified environmental conditions. A basic abnormal occurrence, e.g., burned out bearing in a pump relay not closing properly when voltage applied Fault: Higher−order events, e.g., relay closes at wrong time due to improper functioning of an upstream component. All failures are faults but not all faults are failures. c ✌☎✄☎✍✎✄☎✏✎✠☎✑✝✒✔✓✭✳ ✮✛☎✯✄☎✚✰✜✍✎✏✲✱☎✣✥✄☎☛ ✤✛☎✡☎✤ ☛ ✤✚✰ Reliability Engineering Approach to Safety Reliability: The probability an item will perform its required function in the specified manner over a given time period and under specified or assumed conditions. (Note: Most software−related accidents result from errors in specified requirements or function and deviations from assumed conditions.) Concerned primarily with failures and failure rate reduction Parallel redundancy Standby sparing Safety factors and margins Derating Screening Timed replacements