SPC charts for in-line process control

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    We have an assembly line that we would like to to monitor using SPC charts (defective rate). However, we don’t have an automatic rejects counter, so that the rejects proportion could be significantly different depending on the material that was already processed, therefore the sample size is extremely important.
     I know that If a use a sample size extremely large the chart would allow several rejects prior to have an out of control point, as well as a small sample size would increase the false alarms would be incremented. Do you know any statistical approach that would be helpful for calculatng the sample size needed for a SPC chart &/or estimating the potential false alarms?
    Best regards 


    Chip Hewette

    Could you please clarify…

    Is there one and only one defect (nonconformance) possible for each item checked?
    Is there an automatic gage that determines if an item is defective or not?
    Are you checking the quantity of rejects in an off-line collection point?
    Are you unaware of the exact production quantity for that number of rejects?


    Eoin Barry

    Sounds like you may be looking for a P-chart – using variable control limits based on sample size. Need more info though – Best of luck,Eoin



    Did you try enough Gemba Kaizen team effort focussed on Poka Yoke implementations targeting 0 defect downstream before implementing a control chart that is a monitoring tool ?
    May be you will not need a control chart any more after Gemba Kaizen improvements…

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