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Rational Subgrouping headache

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    DD
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    we coat a test part alongside identical production parts for a furnace run and then measure the coating thickness in 12 locations around the cross-section after stoving.
    can these 12 data points be considered a rational subgroup and plotted by furnace run on an x-bar – S Chart?
    If the answer is yes, then the x-bar data can be put into minitab and process capability generated.
    That’s all well and good but what happens when one or more of the 12 data points in a subgroup is outside the specification limits?
    The x-bar data might be within spec. limits as it is an average and hence might generate acceptable process capability, but if individual readings are outside spec. limits then surely the process is not able to be classed as capable?
    MBB’s please help here as this doesn’t make sense to me. 

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