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Appropriate for SPC Chart?

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  • #45289

    Lee
    Participant

    In a meat packaging plant meat produced for a packaging line comes in pieces thnt range from about 1.5 lbs to 3+ lbs.  The package spec is for weights from 2.0 to 2.5 lbs, with two pieces per package permitted (one whole piece and a cut piece).  About 80% of the packages need two pieces with the second piece drawn from a pile of meat cut by a second person.  The allowed final range for the production line worker is the same as the specs, 2 to 2.5 lbs., and the weighing is manually accomplished (electronic scales, but no auto reject) prior to packaging.
    There are two shifts, and the operators rotate from day to day and throughout the day to counter boredom.
    Currently QA is using an SPC chart to monitor the weights (here the weight is more set by people than a machine), with a UCL and LCL that are 2.35 lbs and 2.57 [sic] Lbs and based on historical 50 data samples of 5 packages each and is essentially normally distributed (Std Dev. = 0.21 lbs).
    My question: What with the process here being a manual process (as opposed to automated) and with the production allowed range the same as the specifications, it seems to me that the use of an SPC chart is like using an atomic-powered fly swatter to do a simple check on weights.  The production persons generally are non-responsive if the samples are outside of the UCL or LCL, nor when there is a run of values that would call for corrections.  Am I right that for this situation that an SPC chart is the wrong tool to monitor the process?  Recommendations?
     
     
     

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    #147577

    Richard
    Member

    Eugene,
    Your situation appears to have 3 difficulties. 1)  the situation (UCL 2.57, LCL 2.35, Std Dev .21) you describe seems to have some inconsistancies.  2) The operators are not demonstrating they understand the SPC process. 3) the situation isn’t clear to your goals for having SPC at this process.
    1)Assuming the std dev you post is for the whole poulation (e.g all 50 points of 5 pkgs each), the std dev for calculating the control limits would be .21/square root(5) = .0939, the Xdouble bar of your control chart would then be 2.35 + 3*.0939 when calculated from the LCL  = 2.63, which is greater than your UCL.  This has implications that the operators are seeing different results than the SPC model predicts, which leads to 2)
    2)  Operators are generally smater than the credit they generally receive, especially if they have benn performing the process for a long period of time.  Operators intuitively tend to understand control limits and know when an outlier has occured.  The difficulty comes in training them to react correctly to an outlier, especially when the outliers is with their “allowed specification” and management drives productivity/rates.  this drives the operator to allow “in-spec” to meet rates/goals and be seen as performing well.  Management needs to support actions taken resolve out-of-control situations in the same way as out-of-tolerance situations for SPC to provide benefits.
    3) In the most basic sense, SPC is mearly a monitoring tool to inform of deviation from “normal”.  When the “not normal” incident happens, SPC only helps if he data collected about each event contains the clue to the variation that created the “not normal” situation.
    For your process, are you more concerned that the operators have to add a 2nd piece 80% of the time or that your data suggests your prodution is exceddign the 2.5 lbs on a regular basis?  I would think the guidance I could provide would depend on your concern.

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    #147585

    Lee
    Participant

    First, thanks for the reply.
    In regards to the SPC chart limits:  You did check the problem before you replied, and caught an issue.  I usually think in terms of the Std Dev. of the packages, so the quoted Std. Dev. was from the 250 individual package weights, and is 0.212 Lbs.  The Std. Dev. of the 50 sets of 5 is 0.117.  To construct the SPC chart I used UCL=A2*R+Ave, where A2 is 0.577 for groups of 5, R is the average Range (0.3724 for the 50 sets), and the average of the 50 averages is 2.349, thus UCL=2.564
    As for the operators:  I agree that they are smarter than they are given credit — it amazes me that we hire people that are involved in civic duties, that run a household and pay taxes, yet when they clock in they are believed to be incapable of thought!  Sometimes I think that a process improvement would be to remove the time clocks.  Anyway, in my case the operators are not seasoned persons, some have been at the job as little as two days, some as long as 6 months, fewer beyond that.  I’m tending to think that believe that better operator training is the stone we need to step on to move forward.
    As for the area of concern:  My concern is producing product that is outside of the 2 to 2.5 Lb range.  The customer specs allow two pieces to be inserted -so using two pieces is not a deviation.  Of the 250 individual packages weighed (data taken from the raw SPC chart data), 41 packages in fact were not within the spec. range.  What I’m fighting (I think) is the mindset of “its within specs, so it’s ok”  vs “the process changed so lets find out what happened”.  The lingering question that I had is if an SPC chart is still applicable in a process that is dominated by human behavior rather than machine performance.
    So my line of though is to provide better training on how to do the job, and fight the Goliath of “Its in Spec so its OK”.  The question is “Is the SPC chart applicable for this situation?”

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    #147594

    Richard
    Member

    The answer to the SPC or not for human-based processes vs. machine-based processes is a resounding YES.  SPC is equally valid for any type of process.
    The difficulty lies in the ability to sustain standardized work in a human based process.  This is usually far more difficult than a machine based process.
    Some ideas for helping with the stanardized work is to look for other related metrics (like volume or width or length, etc) that would drive the desired result.  You may find better results physical size of the product than weight (assuming constant density…not sure if it applies to your product..).  I have found great success is finding alternate measurements to drive the desired results.
     

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    #147602

    Lee
    Participant

    I’ll be looking that the process again today and will look for alternate metrics that may be easier to control.  Alternate drivers is a good idea.  Thanks for the help.
    Eugene

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