How Did Reliability Become Confused With MTBF?

When asking for reliability information, do you ask for MTBF?

There's an interesting dance betwen Reliability and MTBF

Our customers, suppliers, and peers seem to confuse reliability information with mean time between failure (MTBF). Why is that?

Is it a convenient shorthand? Maybe I’m the one confused, maybe those asking or expecting MTBF really want to use an inverse of a failure rate. Maybe they aren’t interested in reliability.

MTBF is in military standards. It is in textbooks and journals and component data sheets. MTBF is prevalent.

If one wants to use an inverse simple average to represent the information desired, maybe I have been asking for the wrong information. Given the number of references and formulas using MTBF, from availability to spares stocking, maybe people ask for MTBF because it is necessary for all these other uses.

What I don’t get is why

When someone asks me for the MTBF, I ask them what they want to know.

The standard answer is they want to know the chance an item will survive over some duration. Or they say they want to know the reliability. They ask for MTBF expecting to learn something about an item’s reliability.

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Comments

Efficiency?

I don't see the word efficiency anywhere or am I misunderstanding something

Aren't Reliability and MTBF related?

Hi Fred,

Aren't reliability and MTBF related?

This is what I understand. Please correct me if I am wrong.

Definitions

*] Reliability is a probability, therefore, it must be between 0 and 1 or (0% and 100%). It is unitless.

*] Mean-Time-Between-Failure is not a probability. It has the unit of time, typically hours. It is a measure of life.

Discussion

Assuming that my life measurements are distributed according to the exponential distribution, then once I have calculated the mean value for life (mu) or MTBF, I also have the standard deviation (sigma) for the distribution. With mu and sigma I could calculate k*sigma bounds to capture various proportion of measurements that make up the distribution.

The proportion of measurements, i.e. area under the curve, above some lower specification value is the reliability. No?

If I have MTBF, I could estimate the proportion of life measurements above some minimum life value and thus estimate the reliability. Couldn't I?

Regards,Shrikant Kalegaonkar (twitter: @shrikale, blog: https://shrikale.wordpress.com)