Dawn,

In a given lot of returned material, the relationship between the number of items in that returned lot and the number of nonconforming items found in it by 100% inspection (or sample of the same) tells us nothing, and we cannot calculate any valid ppm from it.

With extra, reliable information - for example, that the returned lot is a random sample of your delivered material - then you can calculate the ppm from the number of nonconformities per the number you inspect. But, if the customer has sent back a portion of your delivery, mixed good and not good, it is much more likely that they have been isolated from the bulk of the material by some sampling plan which ensures that the lot returned will not be a random sample.

To find the actual (and useful) ppm number, you would need to gain the customer's cooperation and be sent valid information as to what the returned material actually represents.

PPM can be misleading and dangerous data if not used carefully. For example, a case I observed where ppm was applied on a daily basis where deliveries were made every day of maybe 200 items. Target is 1000 ppm; Day one zero n/cs from sample of 20 from 200, so zero ppm - good. Day two, the same. Days three, four, five, etc, the same - still good, zero ppm. But day 65 one n/c from sample of 20, which is 50,000 dpm! 50 times greater than the target ppm. PANIC! DISASTER! Find a new supplier! A valid calculation would be 769 ppm, but try expaining that to the 'Find someone to go over and kick that supplier's ass' brigade.

rgds, John C