What is the definition of reliability?

Question

Options

• AThe fraction of time a system is unable to perform its intended function.
• BThe likelihood that a system failure will occur for a specified time range.
• CThe ability of a system to perform its required functions for a specified period of time.
• DThe degree to which a system is accessible when required for use at an instant in time.

• A
  5% (1)
• B
  10% (2)
• C
  80% (16)
• D
  5% (1)

Explanation

Reliability is defined as the ability of a system to perform its required functions for a specified period of time (C), capturing both capability (it must function correctly) and duration (over a defined timeframe).

• A is wrong - describing the fraction of time a system can't function defines unavailability, essentially the inverse of reliability.
• B is wrong - the likelihood of failure over a time range describes unreliability or failure probability, which is related but inverted; reliability focuses on successful operation, not failure.
• D is wrong - accessibility "at an instant in time" is the definition of availability, a point-in-time metric rather than a sustained-performance one.

Memory tip: Think of reliability as a marathon, not a snapshot - it's about whether a system can keep running over time. If the definition mentions a time period and successful function, it's reliability. If it mentions an instant or a fraction, look to availability or unavailability instead.

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