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Reaction Plan for SPC Out of Control

Discussion in 'SPC - Statistical Process Control' started by Pongsakorn, Aug 10, 2015.

  1. Pongsakorn

    Pongsakorn Member

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    There is the audit checklist from customer asking whether the 100% inspection is done on the product when the SPC out of control is found (e.g. point out of UCL/LCL is seen).
    I am not sure if it is necessary since the product is still within the spec.
    Could anyone advise me what should be the statistical explanation to convince customer that it is not necessary to do 100% inspection?
     
    kitisak and maximilian owino like this.
  2. Ravi Khare

    Ravi Khare Member

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    A Control Chart going out of Control only indicates a lack of stability… not necessarily lack of Quality. Two valuable components of SPC are
    1. Process Capability- This will use the histogram to compare the variation and centering of the process with the customer imposed tolerance limits. It will tell you something about the probability that the process running as it does to produce out of specification products. The Cp & Cpk or Pp & Ppk indices are indicators leading to such assessment.
    2. Process Control/ Stability- This will be evaluated by Control Charts. A control chart with control limits applied on the initial study data tells you whether the process is stable as it progresses. Control Limits are not Specification Limits. A Control Chart thus does not talk about the Quality of the Process. The role of a control chart is purely to come up with a loss of stability signal, prompting the process owner to investigate and adjust the process.

    You would perhaps be using the X-bar/ Range Chart for a variable data SPC. A X-bar chart going out of control signals a significant drift in the process centering. A Range chart going out of control signals a significant change in process variation. The centering and variation baseline is considered to be the status of the process when the control limits were applied. A Control Chart going out of control does not necessarily mean that rejection has happened. Conversely a control chart well within control does not necessarily mean all products are acceptable. Rejection can still happen if the initial control limit application is not proper. It is therefore necessary to assure yourself about adequate Pp & Ppk of the process before applying the initial Control Limits.

    The workflow of a typical SPC application can be summarized in the following three steps.

    1. Conduct Initial Process Study. Plot a Control Chart on sub-grouped data. Plot Histogram on the individuals data. No control limits are applied at this stage.
    2. After adequate data has been collected as in step 1, evaluate the Pp & Ppk of the process from the individuals data. If the Process Capability Indices are satisfactory, apply Control Limits. For a good stability to be established, all the points till the stage should fall within the Control Limits applied.
    3. Carry forward the Control Limits applied in the step 2, and monitor the subsequent sub-groups of data against these control limits. Process continuing within the limits can be deemed to be stable. A stable process does not need adjustments and should not be tweaked. Any adjustments made to a process without a Control Chart signal would amount to tampering.
    If you are able to convey to the customer that you are diligently following the above steps and the Statistical reasoning that is discussed here, hopefully they will not insist on a 100% inspection.
     
  3. Pongsakorn

    Pongsakorn Member

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    Dear Ravi, your comment is really helpful. Thank you so much.
    You should also be a good guitar player and singer besides SPC expert. :)
     
    Ravi Khare likes this.
  4. Ravi Khare

    Ravi Khare Member

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    Thank you. I'm more of an enthusiast of all the three I guess :)
     
    Pongsakorn likes this.
  5. reynald

    reynald Member

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    Ravi beat me into answering. :)
    Yes I agree that process capability is the key. If you can show your client that Cpk is still more than 1.33, you can use that as a justification that quality is on an acceptable level. The higher the Cpk, the more pointless it is to do 100% inspection.
     

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