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Statistical process control for precision machining - Part 3 - Sampling Error


Stop X-bar/R Madness!!

It is good to understand what your process looks like over time (as in run chart). One problem with histograms is they can be easily fooled into thinking you have bi-modal distributions, normal distributions and tight distributions, when really you have been victimized by sampling error. Precision machining is clearly a uniform distribution once you have controlled it correctly and have developed the sawtooth curve from the run chart. To get the true distribution in a histogram, your sample must be multiples of complete cycles. If your sample contains any partial cycles, it will not represent the true distribution. This error would affect any statistical software's analysis, and - no matter how much you paid for it - render it useless.

See attachment for resulting historgrams from sampling error. Note, the process distribution is the same, and in control. Some of the resulting histograms may incorrectly lead to one believe that the process was out of control.
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