Calculate PPMs by going back to Date of Production or Date Noticed

Hello! I've been tossing around this thought for some time now, I'd like to know how other quality professionals calculate their ppms. In calculating PPMs do you take your non-conformance back to the date that it was manufactured or keep it in the month that it was found? I'm in retail manufacturing, our complaints that are found after they leave our facility may not be noticed for a few weeks after the store received the product. Therefore a complaint that is returned today very well could have manufacturered back in October. To me it seems like it would be best to go back to the month that it was produced, this would give you an accurate reflection of the state of production at the time. Historically this company hasn't done this, if the return happens today then it would go in this months PPM calculation. Am I over thinking this?

Any input would be greatly appreciated!

Thank you, Melissa

I take it back to the period of manufacture. its' a little more difficult to create the chart adn it does take a bit of explaining at first (I use open dots for the periods where I may still get reports of defects) and closed dots for those periods that are unlikely to have fruther occurences. once teh team gets used to it tehy find that this approach is usually much less confusing and much more insightful and actionable...

Over time I think it will all even out. So why complicate it. If you have to wait a few months for complete data, then what good is it.

Depends on what you are doing with the information. If you are trying to root cause a problem then you need to go back to manufacture date. For tracking most companies use claim date vs production date for simpler tracking and timeliness. If it is for tracking though you could chart both and make a comparison.

Great topc. I have delt with this extensively over the last 15 years. I have tracked the same data both ways. My preferred apprach is to track it both ways. Once the spreadsheets are set-up it really takes no extra time. There are pluses and minuses to both approaches.

Month found
-easy to track and decide which month to put the defects inot.
-The PPM does not change once a month is closed. Simple for management to comprehend.
-does not give true PPM estimate since the defects are tracked against the wrong months's shipments.
-The metric can be consistant if the month-to-month build quantities are stable.

Month Made
-gives a true picture of the process preformance.
-can be more complicated to compile the metrics.
-the monthly PPM values change every month untill every defect is accounted for.
-One benefit is an organizaton tends to stay focused on the big picture since the moth recent month always looks good initially. Helps prevent near-term reactive fire fighting.

I work for a manufacturing company that does engineer to order machines. Our PPM calculation is based on when detected.

Currently we include all nonconformance reports issued regardless of the final disposition.

What are everyone's thoughts on including the issues that were dispositioned "Use As Is"? Currently they are included as part of the PPM calculation.

Personal choice.

But personally, excluding them would never be OK with me since it was still a defect. The desposition decision is another matter. Think about it, you could simply decide to use all defects "as is" and then conclude you have perfect quality (0 PPM). From a different perspective, if "use as is" is a frequent conclusion then the specs should be reviewed.

Thank you for your insight.