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, ASQ Quality Press, 2000, page 333.