It’s not MTBF. It’s not just the period of time the product does not fail. It’s not just a probability.
It’s a bit more. Reliability is it ‘just works’.
HP calculators are reliable. They work and keep on working. Apparently Lexus makes reliable cars. (According to the current car rankings by Consumer Reports, 2015). My coffee maker is reliable.
The dictionary on my Mac says reliable is:
And, according to O’Connor and Kleyner in Practical Reliability Engineering, 5th ed. Reliability is:
The probability that an item will perform a required function without failure under stated conditions for a stated period of time.
This is a definition we can use as engineers. It has four parts:
And we certainly can define and measure each well.
BTW: MTBF is only probability (actually stated as an inverse failure rate), thus does not fully define reliability.
Consistent, trustworthy? Yes, a reliable product or system should process these essential qualities, too.
Reliability conjures many images and thoughts. The examples you envision are different than mine. That is fine. The concept remains the same. When an item is reliable, it just works. I like to add that it just keeps on working.
When setting goals, estimating, predicting, or measuring reliability, use all four element of the definition laid out by O’Connor and Kleyner. Be clear and complete. Keep it simple and make it reliable.
What comes to mind when you think of reliability? Leave a comment and share what you consider reliable.