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Learn About Quality

What is Reliability?


Quality Glossary Definition: Reliability

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

The most important components of this definition must be clearly understood to fully know how reliability in a product or service is established:

  • 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 can be seen as:

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

Components of Reliability
Components of Reliability

Common examples of product reliability statements or guarantees include:

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

Quality vs. Reliability

Reliability has sometimes been classified as "how quality changes over time." The difference between quality and reliability is that quality shows how well an object performs its proper function, while reliability shows how well this object maintains its original level of quality over time, through various conditions.

For example, a quality vehicle that is safe, fuel efficient, and easy to operate may be considered high quality. If this car continues to meet this criterion for several years, and performs well and remains safe even when driven in inclement weather, it may be considered reliable.

Asking a few key questions can help one determine the difference between both quality and reliability:

  • Quality = Does the object perform its intended function? If so, how well does it perform its intended function?
  • Reliability = To what level has said object maintained this level of quality over time?

Reliability resources

The Importance of Reliability and Customization From Goods to Services (PDF) Although there’s a substantial body of research on quality, disagreement remains as to the effect of reliability, or things gone wrong, as opposed to customization, or things gone right, on customer satisfaction with goods and services.

Estimating Interrater Reliability of Examiner Scoring for a State Quality Award (PDF) Examiner scores for two years of a state quality award were analyzed by sector to estimate interrater reliability.

Reliability Sample Testing—A Case History (PDF) While this article was written in the early 1970s, the rules that applied to reliability sample testing still apply today.

Reliability Engineer Certification

A Certified Reliability Engineer (CRE) is a professional who understands the principles of performance evaluation and prediction to improve product/systems safety, reliability and maintainability. Learn more.


Adapted from The Desk Reference of Statistical Quality MethodsASQ Quality Press.

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