In the past, I was calculating final DPPM based on our final inspection of our process taking into account the following variables,
Total parts/lots produced per time period
Number of defects
using the formula DPPM=[(Number of defects)/(Total Oportunities)]*1,000,000
To be in the proactive side we need to move our inspection/monitoring from final to in-process so that we catch problems before our customer get them.
The problem I’m running into is how to calculate DPPM on in-process inspection enviroment since the above equation takes into consideration Final Inspection Total Oportunites.
Final Inspection Total Oportunities was as follow,
TO= (Sample Size per lot)(Number of lots inspected)
Say we have 10 lots of parts. Our procedures calls to inspect 12 parts on every lot. Then, the total oportunities for these 10 lots is 120.
Could someone help me figure how to calculate DPPM on in-process enviroment? In other words, if we have an endless running process and basically there is not a definite lot size, how can Total Oportunities be calculated?
I would very much appreciate any comment/help on this issue.
1) You are using DPMO
2) One part may have more than one Opportunities
3) You can use “DPMO= ((Total Defects) / (Total Opportunities)) * 1,000,000″ to calculate DPMOs in Process Level, Work Order Level, Product Line level and Overall level.
follow up Mr. Logan Luo is correct to inform you that is DPMO.
After you calculate that ,your process show about the Defect per unit and oppertunity in unit PPM (Part per milion).
So that is not a Z level.We call Z Bench (Area of defect)
by Excel program fx=NORMSINV(1-(XX)) you can calculate the Z Bench to Z- level by the result plus 1.5 (Z-shift)
Sounds like long term and short term sigma.
I also use a short cut to calculate sigma level “process Sigma = 0.8406 + SQRT(29.37 – 2.221 * (ln(DPMO)))”. Using this short cut, I can easily calculate all these measurements (DPU, Sigma, DPMO, Yield and etc.) in my database.
To answer Sergio’s question, I think the key is to calculate DPMO in different levels/process phases, and to have process performance indices in different levels. (E.g. work order/lot, product l, and overall level).
Like you did in final inspection, you can calculate DPMOs in lot, product, and overall levels to represent the performance of your process, product and plant’s quality level.
Does anyone have experience in using similar approach monitoring process performance?
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