Quality = Does it perform its intended function?

Reliability = How long will it do so?

Reliability can be defined as the probability that a product, system or service will perform its intended function adequately for a specified period of time, operating in a defined operating environment without failure.

Components of this definition:

  • Probability: the likelihood of mission success.
  • Intended function: for example, to light, cut, rotate or heat.
  • Satisfactory: perform according to a specification, with an acceptable degree of compliance.
  • Specific period of time: minutes, days, months or number of cycles.
  • Specified conditions: for example, temperature, speed or pressure.

Stated another way, reliability is

  • Probability of success
  • Durability
  • Dependability
  • Quality over time
  • Availability to perform a function

Common examples:

  • “This car is under warranty for 40,000 miles or 3 years, whichever comes first.”
  • “This mower has a lifetime guarantee.”

Excerpted from Mark L. Crossley, The Desk Reference of Statistical Quality Methods, page 391.

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