![]() ![]() Again, multiplying these indexes by 3 yields their individual sigma levels. This however is a faulty assumption especially for processes that are dramatically shifted to one side or other of the specification range.Ī more precise estimation of PPM would require your breaking up Cpk (or Ppk) into its constituents, Cpl and Cpu (Ppl and Ppu). Multiplying this output by 2 assumes that both sides of the curve yield an equal portion of parts out of specification, resulting in an approximation of the portion defective. Afterall, the Cpk and Ppk indexes only consider the specification limit closest to the process mean the opposite side is discarded in the equation. Now the astute observer may already see a problem with this approach:ġ-NORM.S.DIST(Z,TRUE) gets you to the cumulative portion of defective parts on one side of the curve, namely the side where the specification limit is closest to the arithmetic average of the process. To get from the sigma level to an approximate PPM in Excel, you use this formula: a Cpk of 1.67 equates to a 5 sigma process.) It’s easy to see from the table that the “sigma level” is 3 times the capability index (e.g. To approximate the PPM for your specific Cpk level, you can shift over to Microsoft Excel. Using this table, we can see that a Cpk (or Ppk) of 1.33 equates to a +/-4 sigma process which yield approximately 63 defects per million parts produced. One of the most common questions I get from students in my Process Capability Class is, how can I use the capability index from my process to approximate a defect level for my process?įor most, the answer comes from using a table like this one: Root Cause Analysis and the 8D Corrective Action Process course.An Introduction to Reliability Engineering.Reliability Analysis Methods online course.14 Ways to Acquire Reliability Engineering Knowledge.Reliability Engineering Management DRAFT.Innovative Thinking in Reliability and Durability.Equipment Risk and Reliability in Downhole Applications.Musings on Reliability and Maintenance Topics.Metals Engineering and Product Reliability. ![]()
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