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  • #50949

    Tinoco
    Participant

    When a process is capable can it be improved, if so how? And what does ‘out of control’ actually mean? As an operator what needs to be done to bring the process in control? Is there a point in a control chart where one would say it’s too out of control to recalculate control limits.

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    #175809

    Mike Carnell
    Participant

    Anthony,
    When you ask a question like “And what does ‘out of control’ actually mean?” You need to do some reading on your own. If there is one thing quality people love to talk about it is control charts.
    Start with the new to Six Sigma on the left side of the screen and then a couple books by Wheeler.
    Just my opinion.

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    #175813

    Mikel
    Member

    I agree with Mike’s message, but let me see if I can at least give you some things to think about –
    When a process is capable can it be improved, if so how?
    So the obvious question here is what is your definition of capable? Cpk > 2; Ppk > 1.5; Cpm > 1.33, …?
    The simple answer is yes, any process can be improved, the question is which process should you be improving,
    And what does ‘out of control’ actually mean?
    In the simplest terms, it means it is not predictable.
    As an operator what needs to be done to bring the process in control?
    An operator needs to know what things are in their control and how to tell from the data if what they are seeing is controllable by them.
    Is there a point in a control chart where one would say it’s too out of control to recalculate control limits.
    You never ever recalculate because the process has become unprdictable.
     
     

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    #175823

    MikeAjr
    Participant

    When a process is capable can it be improved, if so how?
    Unless you have completely eliminated all variation in a process (you never will) it can be improved.  How you improve it depends on what type of variation you are seeing.  Special cause variation is usually very easy to reduce/eliminate.  Common cause variation (variation you would expect to see) can be very difficult to reduce.
    And what does ‘out of control’ actually mean?
    Special cause variation has been introduced into the process. (Examples – loose bolt, broken tooling, etc.)
    As an operator what needs to be done to bring the process in control?
    Perform root cause analysis and / or utilize the DMAIC process.
    Is there a point in a control chart where one would say it’s too out of control to recalculate control limits.
    Would your customer be pleased to know that you recalculate control limits because you gave up on solving the issue at hand?

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    #175824

    Severino
    Participant

    I have to clarify one point you are making here Mike.  You should not be reducing or eliminating common cause variation.  If you have improved the process to the point where your common causes have become special causes that is one thing, but reaction to common cause variation will only result in your process spiraling out of control.

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    #175827

    Vallee
    Participant

    Jsev607,
    While I agree that you should not react (knee-jerk) to common variation, I would not say that you should not attempt to reduce the spread of common variation.  You reduce variation by proactively reducing generic (systemic issues) through continuous improvements.  One way is through audits in high safety and business risk processes. You don’t have to wait for an incident.
    HF Chris Vallee

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    #175832

    Severino
    Participant

    I guess my feeling on it is that a common cause is a source of variation that happens at random for which you are unable to identify a direct source.  If you audit a high business/safety risk process and you identify an activity or condition which contributes to your variation you have picked out an ‘assignable’ (special) cause.  The distinction is purely academic and probably subject to debate. 
    In the end I would never argue against reducing variation, my intent was to ensure that Mike didn’t start making adjustments based on every data point.

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    #175837

    Vallee
    Participant

    I would keep true to the original definitions of special and common cause and not call items found in an audit special cause unless they are out of whack indicating something in the system changed.  Risk audits can be guided by two factors (keeping it simple): the top drivers (leading indicators) or areas that have a high risk level. 
    Look at reducing common cause variation as proactive and special cause as reactive. Reactive can also reduce other near misses and system problems if this probem is not just a one off problem which also can reduce common variation spread.
    HF Chris Vallee
     
     

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    #175842

    Craig
    Participant

    Some good answers already, and here are more tidbits.
    When a process is capable can it be improved, if so how?
    A: Widen the spec limits (Just Kidding) B: decompose the variability and see if the biggest source is within piece, piece to piece, or temporal…then look for optimization alternatives. Obviously you have to prioritize your improvement opportunities before jumping the gun.
    And what does ‘out of control’ actually mean?
    A) A process that I designed (Just kidding) B) Look at the control chart (half kidding) C) Stan hit the nail on the head on this one. Unpredictable.
    As an operator what needs to be done to bring the process in control?
    Ressearch what is meant by OCAP. Hopefully the chart designer thought about this before rolling the SPC program out to the floor
    Is there a point in a control chart where one would say it’s too out of control to recalculate control limits.
    If your process is unstable to a great degree, you have no business using SPC to begin with. SPC charts can be used as part of characterization, but on-line SPC with a good selection of rules and some well written OCAPs are really targeted for the control phase.

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