PPM indicates the number of nonconforming parts in your process, expressed in parts per million. For example, a manager of a fast food restaurant observes that 20 out of 1000 customers waited more than 3 minutes for service. Therefore, PPM = (20/1000) * 1000000 = 20000. This means that 20,000 customers per million customers wait for more than 3 minutes for service.
Although Cpk and Ppk are the most commonly used metrics for process capability, they measure how the process is performing only in relation to the specification limit that is closest to the process mean. Therefore they evaluate only one side of the process curve, and do not directly indicate how the process performs on the other side of the process curve.
To get a clearer indication of how the process is performing on both sides of the process curve, you can use other indices, such as PPM. PPM < LSL indicates the number of nonconforming parts less than the lower specification limit. PPM > USL indicates the number of nonconforming parts greater than the upper specification limit. PPM Total indicates the total number of nonconforming parts outside of both specification limits. By comparing PPM values before and after a process improvement, you can get a concrete sense of how much a process improvement has actually reduced the number of nonconforming parts on both sides of the process curve.