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
- Quality over time
- Availability to perform a function
- “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.