Your data is simply out of control not sure if its Non-Normal as Mr Haynes said or not, but certainly out of control. Have a look at what lies behind the Estimate button and use the Help button to drill down to Methods and Formulae for all the gory details. The estimate of sigma you used is essentially what it refers to as StDev(Between). It refers to this estimate of sigma as StDev(Within). The mean moving range divided by the constant d2 (1.128 for ranges based on samples of two measurements) gives an estimate of the process sigma which leads to the value you got from Minitab. Copy this formula down the column to get all 29 moving ranges. Once two have been measured the formula ABS(B3-B2) in cell C3 gives the first moving range of 0.02. (See below.) With only one part measured no moving range can be calculated. Use the estimate of standard deviation obtained via the mean moving range method. Where A1 is the cell that contains the process averageĪ2 is the cell that contains the process standard deviationĪ3 is the cell that contains the Upper Spec LimitĪ4 is the cell that contains the Lower Spec Limit. Have worked out Cpk long hand in excel – Cpk = 0.71 see belowĪnd get two quite different results – why ?
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