There are a couple commonly used Microsoft Excel functions that convert Defects per Million Opportunities (DPMO) to a process sigma ( z-score) and vice versa. A negative sigma value means that most of the process is performing outside your customer's specification range (LSL and USL). Z-table for 0.525 is less than 0 sigma, long term. The probability of having a defect in the entire series is: This is considered a long term sigma value.įor an entire series of processes use the RTY in place of TPY above. Z-table for 0.20 is approximately 0.84 sigma. Simply says that 100% minus the % of not having a defect from either scrap or rework in ONLY Process 1 is the probably of having a defect. Because above 10,5 or below 9.5, not only it is less aesthetical but it also because dangerous for the user.The probability of having a defect in Process 1: For example we may allow a PPM rate of 100 on 10+/-0.1 because it has an aesthetical impact and then a PPM of 10 (out of the 100) above 10+/-0,5. E.g.: On a single functional criteria we may have multiple conditions. We can also note that in some industries, depending on how much outside specification the function is, it can addtionnal PPM requirements. The higher the stack severity is the lower the PPM must be. We refer to it as Part Per Million but we focus on the product functionnal criteria outside specifications not only on parts. So the use of PPM assumes a statistical approach on the product and process dispersion contributors. The PPM defines how many Parts per Million are tolerated to be outside specification by the customer.
From the FMEA an objective PPM rate is set for each functional criteria. This data obtained during the functional analysis phase is stored in the FMEA. At the beginning of any project, the customer expectations are turned into measurable data.
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