Question
My question concerns the process performance metric DPMO (defects per million opportunities). I want to use this to quantify a particular supplier’s performance. My question is, is the number of defects referred to in the calculation the number of defects produced by the supplier (in which case it would involve data I don’t have access to), or is it the number of defects experienced by the customer (which is us)? I of course can count the number of defects we receive from the supplier, but if this metric is supposed to be based on the number of defects produced by an organization, I would have no way of knowing how many defects are produced by the supplier’s process, but contained within the supplier’s facility. My hope is to be able to characterize the supplier’s process performance in terms of sigma level.
Answer
The DPMO metric is not usually considered a point estimate of the true percent defective in the lot (either at the supplier or customer site). It is a relative performance metric used to equate the observed percent defective from a sample to defective units per million opportunities. If a supplier culls out all the defective units before shipping to you (i.e. perfect inspection system), your internal DPMO would be 0, even if the supplier DPMO is high. If your goal is to characterize the supplier’s process performance in terms of sigma level, you would need their data, as the data you collect internally is just an estimate for the average outgoing quality from the supplier and not their process performance.
For more on this topic, please visit ASQ’s website.