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Process Capability vs. Process Reliability

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  • #34301

    Info Seeker
    Participant

    Is there a difference between process capability and process reliability or are they the same thing?
    Seeker

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    #94252

    Rog
    Member

    It depends on what you are measuring. Process capability or reliability is a measure of how capable your process is to deliver a specified outcome. Traditionally Process Capability would be the term used with respect to a product quality output. I would expect most process managers and engineers would link the term ‘reliability’ with the ability of the process to operate sustainably. The process may show a high process capability when it operates but a low process reliability if it only operates for 50% of the time it is supposed to. It is really a semantic difference though.
    One could use the same statistical tools to monitor each. My only caution, from a Six Sigma sense, would be how many causes of process interruption actually arise from statistical variation and how many are assignable causes ?
     
    Rog

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    #94271

    Michael Ervick
    Participant

    I have always liked the definition offered in the text book: Reliability: Probabilistic Models and Statistical Methods by Dr. Lawrence M. Leemis, Department of Mathematics, College of William and Mary.
     
    “The reliability of an item is the probability that it will adequately perform its specified purpose for a specified period of time under specified environmental conditions.”
     
    The best for capability is quilted together from information provided by Frank M. Gryna’s famed text; Quality Planning and Analysis: From Product Development Through Use.
     
    “The capability of a person, process or organization is the ability, based on tested performance, to achieve measurable results that satisfy established requirements or specifications.”
     
    Traditionally reliability is time oriented performance quantified in terms of probability.  Capability is event oriented performance quantified in terms of probability.  Reliability tends to be a performance metric assigned to mechanisms or systems without the variable of free will.  Capability tends to be a performance metric assigned to individuals and organizations with the variable of free will.
     
    While this would tend to explain why machines are more reliable than humans, and why humans are more capable than machines, it does not explain why processes tends to use both reliability and capability interchangeably.
     
    Michael Ervick

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    #94298

    Gabriel
    Participant

    Capability: Ability to do it right.
    Reliability: Ability to do it right, agin, and agin, and again…

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    #94301

    Vyomesh
    Member

    All Processes which are reliable are Capable of delivering what they have been designed for. Reliability has a built in Capablity. Rather Capablity with dependablity to deliver the needful makes a Process Relaible.

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    #94310

    Michael Ervick
    Participant

    So are we saying that
     “Availability” X “Capability” X “Repeatability” = “Reliability” 
    Michael

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    #179065

    Sayyed
    Participant

    So, can the term Reliability be used interchangebly with Stability. In other words process stability can be measured using Control Charts & Run Charts while Capability by Cp, Cpk? Or is Reliability different from stability (because stability does not consider customer spec limits)?
    Neal
     

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    #179081

    annon
    Participant

    Too much free time…just use stable and capable and avoid the confusion.  The former deals with consistency and the latter with meeting an expectation.

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