What is the best definition of reliability?

R. Keith Mobley

R. Keith Mobley

Executive Advisor, Domain Expertise, Operating Dynamics Modeling, Predictive Analytics

Published Feb 17, 2020

Over the past few weeks, we have had a running dialogue about failures and related topics. Perhaps its time to tackle reliability.

If you asked one hundred people to define reliability, ninety-nine would respond with something like, “The ability of an item to perform a required function under stated conditions for a specified period of time.” This definition can be sources to the Dictionary of Military and Associate Terms, published by the Department of Defense in 2005. Since then it has morphed into the go-to definition for any discussion of reliability, but is this the real definition of reliability?

If you look up the root word of reliability, the definition is “Consistently good in quality or performance; able to be trusted. A person or thing with trustworthy qualities.  It also means dependable, well-founded, authentic, valid, genuine, trustworthy, committed, unfailing, infallible, and constant. The definitions are the same, but when interpreted carry different meanings.

Since the majority of readers are focused on capital or tangible assets, let me couch true reliability in this way. First, we will stipulate that your assets are perfectly designed for reliability; have been installed and commissioned with absolute adherence to best practices. Now what must you do to sustain their reliability and prolong that reliability for their full economic useful life.

I am reasonably sure that you would respond with proper operation and maintenance are necessary, but is that enough? What about the production planning and scheduling function’s decision to run a non-standard product that will cause significant damage; or the sales department promising short delivery that forces, production scheduling to defer maintenance windows; or procurement’s decision to substitute non-spec. materials or parts; or the executive leadership’s decision to reduce operating costs and headcount.

Do any of these decisions impact the inherent reliability of the tangible assets? Of course, they do, but when we discuss reliability decision-making and the way work is planned, scheduled and executed is not included in the conversation. The only way that a plant, company or enterprise can be reliable is when everything from the way individual employees think through how they work together as a team must be consistent, repeatable, well-founded—in other words reliable.