Reliability metrics Flashcards
Describe Probability of failure on demand (POFOD).
If you use this metric, you define the probability that a demand for service from a system will result in a system failure. So, POFOD = 0.001 means that there is a 1/1000 chance that a failure will occur when a demand is made.
When should POFOD be used?
POFOD should be used in situations where a failure on demand can lead to a serious system failure. This applies irrespective of the frequency of the demands. For example, a protection system that monitors a chemical reactor and shuts down the reaction if it is overheating should have its reliability specified using POFOD.
Describe Rate of occurrence of failures (ROCOF).
This metric sets out the probable number of system failures that are likely to be observed relative to a certain time period (e.g., an hour), or to the number of system executions. For instance, the ROCOF could be 1/1000.
When should ROCOF be used?
ROCOF should be used when demands on systems are made regularly rather than intermittently. For example, in a system that handles a large number of transactions, you may specify a ROCOF of 10 failures per day.
Describe mean time to failure (MTTF).
The reciprocal of ROCOF is the mean time to failure (MTTF), which is sometimes used as a reliability metric. MTTF is the
average number of time units between observed system failures.
When should MTTF be used?
If the absolute time between failures is important, you may specify the reliability as the mean time to failures (MTTF). For example, if you are specifying the required reliability for a system with long transactions (such as a computer-aided design system), you should use this metric.
Describe Availability (AVAIL).
AVAIL is the probability that a system will be operational when a demand is made for service. Therefore, an availability of 0.9999 means that, on average, the system will be available for 99.99% of the operating time.
