"X-MR charts do not separate piece-to-piece repeatability of the process"

[stm08007]

Hello,

Still studying for the CQE and have another basic question that I am getting hung up on. When the Primer talks about pros and cons of X-MR control charts (relative to other control charts), one negative is that they do not separate piece-to-piece variation... To me, they do this more than any other chart since they are literally telling you a moving range and the actual measured value of two subsequent pieces.

Does anyone understand this statement?

 

[Slim Pickins]

If you were to measure one piece every two hours and chart it, you can see the long term process variation (X-MR). IF you were to measure 5 pieces every two hours you would be able to see the short (part to part) and long term variation. (XbarR)

 

[stm08007]

If you were to measure one piece every two hours and chart it, you can see the long term process variation (X-MR). IF you were to measure 5 pieces every two hours you would be able to see the short (part to part) and long term variation. (XbarR)

Ah that's kind of what I was thinking, though the book seemed to imply that X-MR was used when there were not many data points (i.e. 5 pieces are built each hour and every part is checked)-- if that was the case, I assume you would see the short part-to-part variation. Your answer makes sense though!