• Basic definition - Information that can be depended upon with confident certainty; absence of NO error, breakdown, or or performance.[1]
  • Technology definition - The trustworthiness to do what the system is expected or designed to do.[2] Any system that consistently produces the same results[1]
  • Computer Networking definition - A reliable protocol ensuring reliability properties with respect to the delivery of data to the intended recipient(s), as opposed to unreliable protocol, which does not guarantee that data will be delivered intact, or that it will be delivered at all.[2]
  • Layman's definition - How reliable is it?
  • reliable – giving the same result after repeated trials / usages
  • Explanation to a 12 year old - The probability that something will do what it is supposed to do.

Different types of reliability in technology

  • POFOD (probability of failure on demand):
  • The likelihood that the system will fail when a user requests service. A biometric authentication device that fails to correctly identify or reject users an average of once out of a hundred times has a POFOD of 1%.[2]
  • ROCOF (rate of failure occurrence):
  • The number of unexpected events over a particular time of operation. A firewall that crashes an average of five times every 1,000 hours has a ROCOF of 5 per 1,000 hours.[2]
  • MTTF (mean time to failure):
  • The average time between unexpected events. If an IDS fails on average every 300 hours, its MTTF is 300 hours.[2]
  • AVAIL (availability or uptime):
  • The percentage of time that a system is available for use, taking into account planned and unplanned downtime. If a system is down an average of four hours out of 100 hours of operation, its AVAIL is 96%.[2]

Importance of reliability

Now that technology and computers are so often used in an innumerable amount of places, reliability is becoming a critical aspect of information technology in our society.

For example, the software used in airplanes and air traffic control centers must be extremely reliable, considering the fact that many lives are in grave danger in the event of a failure.

Events involving reliability

  • In 1999, the Mars Climate Orbiter crashed because the programmers who wrote the code for the software neglected to translate English measurement units to metric units.[3]
  • An new Airbus A320 crashed at an airshow in France on June 26, 1988 because of faulty software that caused his altimeter to not respond properly resulting in the pilot crashing into trees, killing the three passengers in his plane.[4]
  • Therac-25, a computerized radiation therapy machine for cancer patients, was the cause of six accidents, ending in death or critical injury, due to the software wrongly administering an overdose.[5]


Nick Apseloff and Ricardo Aldana