The number of nonconforming parts out of a million parts. For example, a manager of a fast food restaurant observes that 20 out of 1000 customers waited more than 3 minutes for service.
|
PPM = |
20 * 1000000 |
= 20000 |
|
1000 |
This means that 20,000 customers per million customers wait for more than 3 minutes for service.
It can be easier to understand the amount of improvement in capability when it is expressed in PPM rather than as a capability index. Suppose a process is improved and its Cpk increases from 1 to 1.33. This change may not clearly indicate the degree of improvement. However, you can also express this improvement as a significant decrease in PPM from 66807 PPM to 6210 PPM.
|
Percentage of Conforming Parts |
PPM |
Cpk |
|
93 |
66807 |
1.00 |
|
99 |
6210 |
1.33 |
|
99.977 |
233 |
1.67 |
|
99.9997 |
3.4 |
2.00 |
|
|
|
|
Minitab expresses the expected and observed performance of the process in terms of PPM. The expected performance is the expected number of nonconforming parts with respect to the upper, lower, and both the specification limits out of a million parts. The observed performance is the actual number of nonconforming parts with respect to the upper, lower, and both specifications out of million parts in the process.