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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

You can also search articles, case studies, and publications for reliability resources.


Practical Reliability Engineering

The Certified Reliability Engineer Handbook

Practical Engineering, Process, And Reliability Statistics


My First Steps In Quality And Reliability: Lessons From The Russian Space Program (Quality Progress) "Reliability is a pseudoscience, akin to astrology!" With these words, I was greeted by the operations manager for the Soyuz ILV Complex in Moscow when I ask him for his last five years of missile failure data. This was my first day on the job as a reliability engineer.

An Enhanced Parenting Process: Predicting Reliability In Product's Design Phase (Quality Engineering) The design for reliability philosophy emphasizes early reliability predictions at stages for which no reliability data has been generated. This article proposes an enhanced parenting process, a rigorous series of mathematical formulations that produce a statistical inference of the failure rate. An example demonstrates the method.

Case Studies

Rainbow SPC Process: Using Statistical Tools For Accelerated Product Development And Enhanced Reliability (PDF) Traditional approaches to statistical process control charting are effective when it comes to monitoring process behaviors and providing useful data for continuous improvement efforts. However, many of the same tools typically reserved for inspection and basic quality control can have much broader applications for enhanced quality and reliability.

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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