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What Is Reliability?

Depending upon your perspective, there are multiple answers to this seemingly unanswerable question.

Let us look at a few:

Reliability Engineer Definition

Reliability is the probability that a device will perform its intended function during a specified period of time and under stated conditions. Mathematically, this may be expressed as R(t) = Pr{T>t}.

This means that failure is regarded as a random phenomenon: it is a recurring event and we do not express any information on individual failures, the causes of failures or relationships between failures, except that the likelihood for failures to occur varies over time according to the given probability function.

Reliability is predicated on “intended function.” This is taken to mean operations without failure.

Reliability applies to a specific period of time. In practical terms, this means that a system has a specified chance that it will operate without failure before time (t).

Reliability is restricted to operation under stated, explicitly defined conditions. This constraint is necessary because it is impossible to design a system for unlimited conditions.

Department of Defense (DOD) Definition

Reliability is the probability that an item will perform a required function without failure under stated conditions for a stated period of time.

American Society of Quality (ASQ) Definition

Reliability is defined as the probability that a product, system, process, or service will perform its intended function adequately for a specified period of time, or will operate in a defined environment without failure. The most important components of this definition must be clearly understood to fully know how reliability in a product or service is established:

  • Probability: the likelihood of mission success
  • Intended Function: for example, to light, cut, rotate, heat
  • Satisfactory: perform according to a specification, with an acceptable degree of compliance
  • Specified Period of Time: minutes, days, months, or number of cycles
  • Specified Conditions: for example, temperature, speed, or pressure

Stated another way, reliability can be seen as:

  • Probability of Success
  • Durability
  • Dependability
  • Quality Over Time
  • Availability to Perform a Function

Various Dictionary Definitions

  1. Reliability means being able to depend on or trust someone or something.
  2. The extent to which an experiment, test, or measuring procedure yields the same results.
  3. Yielding the same or compatible results.
  4. In statistics, the tendency of a system to be resistant to failure.
  5. The quality of being trustworthy or of performing consistently well.

Most focus on the DOD definition and actually flip reliability as the probability of a tangible asset’s failure (the inverse) rather than that of survival. A reliability probability of 95% the reciprocal of a 5% probability of failure. Other key points in the way most are using reliability is that the focus is on the failure modes and forcing functions, but the ASQ definition explicitly states that these factors are not considered in the probability calculation. Instead, all failures without discrimination or descriptions are combined.

Where this DOD approach failures is that it does not consider the impact of reliability or lack of reliability throughout our society, culture, personal lives as well as the enterprise organization. Consider the synonyms for reliability:

  • Dependability
  • Consistency
  • Responsibility
  • Trustworthiness

Now, think about what the fundamental requirements of an enterprise organization. It depends on decision-making, planning, scheduling, management and work execution that are reliable, e.g. dependable, consistent, repeatable, etc., from the boardroom to the factory floor. The instability, inconsistency and variability in these crucial process that is common today, is the reason too many organizations are reactive and will remain so.

"What is Reliability?" is one the enquiries from the Certified Reliability Leader ® workshop offered by® at various dates throughout the year. You can find current scheduled dates listed on the event calendar.

Intentional Reliability LeadershipIntentional Reliability LeadershipTerrence O'Hanlon

R. Keith Mobley, CMRP, MBB

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