During RAMS this year, Wayne Nelson made the point that language matters. One specific example was the substitution of ‘convincing’ for ‘statistically significant’ in an effort to clearly convey the ability of a test result to sway the reader. For example ‘the test data clearly demonstrates…’
As reliability professionals let’s say what we mean in a clear and unambiguous manner.
Thus, you may suspect, this topic is related to MTBF.
Simply saying ‘reliability’ instead of ‘MTBF’ would convey what we really mean. If the message requires specific values, instead of ’50,000 hour MTBF’, say ’98% reliable over two years’. And, if you absolutely have to use MTBF, always add the duration over which the failure rate (1/MTBF) is relevant.
MTBF Acknowledged as Misunderstood
During a panel at RAMS, a few panelists spoke of the state of various reliability-related international standards and mentioned the continued use of MTBF. When challenged, which I’ve done on this topic, they defended the continued use of MTBF due to its widespread use. They also acknowledged the common misunderstanding and misuse of MTBF and agreed that using ‘reliability’ is more meaningful. Yet, they contended that the overall widespread use of MTBF warranted the continued use in the standard’s language.
I contend that the standards establish and reinforce the language that we use as a profession because language matters. We should expect the language of any standards to be clear and easy to understand. While MTBF, in itself, is a perfectly meaningful expression, when used correctly, it does not currently communicate the intended message. Changing the term ‘MTBF’ to ‘Reliability’ in standards would encourage our profession and those that rely on reliability standards to elevate the discussion, to speak and write clearly, and to avoid the communication errors surrounding MTBF.
Reliability-related standards further propagate the acknowledged widespread misunderstanding by the repeated appearance of standards. Let’s change that.
Objection and Objective
One objection is the ‘we’ve always done it that way’ type argument. This is related to the early medical profession’s use of Latin to cloak the professional in an aura of professionalism (educated elite members reinforced with arcane language). A major objective of the reliability profession is to enable the design and management teams to make good decisions while considering the full impact of reliability. Using ‘arcane’ or difficult-to-understand language does not serve our profession or provide a service to the design and management teams.
We are already facing the daunting task of clearly explaining the range of statistical tools we routinely use to solve problems. Adding the term ‘convincing’ will help in that area. Let’s continue to improve our collective language by avoiding the use of MTBF also.
Thanks. Your article makes it apparent (at least to me) that MTBF is a lie for most products or parts. This is because MTBF requires extrapolation of reliability or failure rate function beyond observed lifetimes: unless the failure rate function is constant.
Theorem from when hell freezes over page 19, Barlow and Proschan, Reliability book, says something like: for a system with a large number of statistically independent components, with identical replacements, the times between failures will, asymptotically, as time goes to infinity, have exponential distributions with mean MTBF.
Meanwhile, consider Credible Reliability Prediction: the prediction of new product reliability using the field reliability and failure rate function estimates of comparable parts that have already been in the field, some of which may have completed their life cycles. Designs, processes, shipping, installation, customers, and environments don’t change much.