When Asking for Reliability Information Do You Ask for MTBF?

Our customers, suppliers, and peers seem to confuse reliability information with MTBF. Why is that?

Is it a convenient shorthand? Maybe I’m the one confused, may those asking or expecting MTBF really want to use an inverse of a failure rate. Maybe they are not 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 asking for MTBF is 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.

Why not just ask for reliability, R(T), the reliability function. The time to failure distribution describing the pattern of failure over time. When I provide R(t) sometimes the next question is “how do I covert this to MTBF?”

Why would anyone want to do that?

References and Habit

Nearly any reference on reliability includes some discussion about MTBF, often an extensive part of a manual, book, or standard only uses examples with MTTF or MTBF. This may be why so many ask for MTBF exclusively.

• Reliability apportionment tools often request MTBF values for subsystems.
• Reliability prediction methods request the MTBF of components (which we then convert to failure rate to add, then convert the result to MTBF).
• Reliability requirements are often written in term of MTBF, which we compare our estimates.
• Reliability testing has formula after formula to determine sample size and test time, two essential elements for planning.

Those that ask for MTBF may be reading only the MTBF related sections of the book, manuals, and standards. For them, the world of reliability engineering revolves around MTBF. So limiting.

“Everyone requests and uses MTBF.” Seems to be habit forming. If you are taught to ‘do’ reliability by asking for MTBF and estimating MTBF, then MTBF is what you ‘do’. It is the way we learn the craft and then teach others. We also seem to stop learning, to stop checking assumptions, to stop noticing the actual results are nothing like the promised MTBF.

MTBF is Not Reliability

Reliability is the probability an item will function over a duration within a specified environment. This is in most if not all reliability related textbooks, glossaries, and standards. Right before the introduction of MTBF.

MTBF is the mean time before (some use between) failure. A common method to estimate MTBF is to calculate the inverse of the average failure rate. Total time divided by the number of failures.

Often when we ask someone about the reliability information of an item, say a flat screen TV, the response is a duration. “The TV has a 5 year life.” That is only a duration and assumes we include a low chance of failure over the 5 years.

If we say the TV has a 5 year MTBF, that is not the same as saying the TV is very likely to survive 5 years. I does mean that on average the TV will failure sometime before or after 5 years and each year (given no other information and assuming a constant hazard rate) the chance of failure is roughly an 18% chance of failure any given year.

Part of the confusion between MTBF and reliability is we often interested in how long an item will survive, a duration, in units of time or cycles. MTBF, since it is units of time (time per failure actually) many confuse MTBF stated in units of time as a duration. The duration over which we expect items to survive.

Duration along is not reliability, it is only one element of reliability. MTBF is actually a probability or the chance of failure per unit time. If the MTBF is stated as 50,000 hours, that literally means there is a 1 in 50,000 chance of failure every hour. It is not a duration. The probability or failure, like the probability of success, is also not reliability. It is just one element of reliability.

MTBF is not reliability. Got it?

Summary

I wonder if the confusion between MTBF and reliability information will really every go away. I have to believe it will, thus continue to maintain the NoMTBF site. With your help, we can clear up this confusion and get on with the important task of creating reliable products that meet our customers actual reliability needs and not their requested MTBF.