Lean Six Sigma Forum
Calculation of Capability (Cp) and Sigma (within)
Hello Michael (and all),
As we discussed on the phone, this pertains to the calculation of Cp (also is related to Cpk, Pp, Ppk) and sigma within page 456 of the Lean Sigma Black Belt pdf file of course material. I used the Minitab file (Capability Analysis) provided in the pdf also. There is a total of 40 data points, n =40 under the column heading "HtBk" with subgroup size of 1. I can easily calcuate the standard deviation overall given in Minitab of 0.314222, but cannot obtain the sigma within value of 0.36052. I used the equation given on page 456 for σ(within) = s(p)/(c4(d - 1)). For the calculation of c4 I used ((4*(40-1))/(4*30-1) = 0.9936. However, the formula for d is confusing as the summation only has a starting point at i. d = summation from i to ? (n(i) - 1?? I attached a screen shot of the page also.
Thanks for your feedback.
Uploaded files:
Hi Stephen,
In looking at this further, d should be n-k+1 which in this case k=1, however, because we already show c4(d+1) we are already accounting for k and therefore d will just be n. Let me know if you can get there with that information. Attached is a slight variation from Minitab support which may help, let me know.
Uploaded files:
Hello,
I am still confused. In Minitab support it gives a table for the c4 unbiasing constant - which I cannot match with the formula from the slides: c4 = (4(n-1)/(4n-3). Is the d not just the sum of the observations minus 1? Thanks