Capability analysis is an essential tool for statistical process control (SPC) and process improvement, applicable to a variety of environments from hospitals and labs to assembly and manufacturing. The formula for calculating a Cpk index is easily found:
USL, upper specification limit; LSL, lower specification limit.
*Estimated sigma = average range/d2
Common understanding includes the fact that Cpk climbs as a process improves – the higher the Cpk, the better the product or process. Using the formula above, it’s easy to calculate Cpk once the mean, standard deviation, and upper and lower specification limits are known.
Only One Specification or Tolerance
But what if you have only one specification or tolerance – for example, an upper, but no lower, tolerance? Is Cpk advisable under these circumstances?
When faced with a missing specification, consider one of the following three options:
- Not calculating Cpk, since all the variables are not known
- Entering an arbitrary specification
- Ignoring the missing specification and calculating Cpk on the only Z-value available
Example: Plastic Pellet Manufacturer
Examining a specific situation may clarify the outcome of each of these possibilities. A customer of a plastic pellet manufacturer has specified that the pellets should have a low amount of moisture content. The lower the moisture content, the better, but no more than 0.5 units is allowed; too much moisture will create manufacturing problems for the customer. The process is in statistical control.
This customer would undoubtedly not be satisfied with option 1 as Cpk has been specifically requested. With option 2, it could be argued that the LSL is 0, since moisture levels below zero are impossible. With a USL at 0.5 and LSL at 0, the Cpk calculation would be as follows.
Assume the X-bar = 0.0025 and estimated sigma is 0.15.
The customer is not likely to be satisfied with a Cpk of 0.005, and that number does not represent the process capability accurately.
Option 3 assumes that the lower specification is missing. Without an LSL, Zlower is missing or nonexistent. Zmin becomes Zupper and Cpk becomes Zupper / 3.
Zupper = 3.316 (from above)
Cpk = 3.316 / 3 = 1.10
A Cpk of 1.10 is more realistic than one of 0.005 for the data given in this example, and is more representative of the process itself.
A Misleading Cpk
As the example demonstrates, setting the lower specification to 0 results in a lower, misleading Cpk. In fact, as the process improves (here, moisture content decreases), the Cpk should have increased. If 0 was used as the LSL, however, the Cpk would have decreased. This is one clue that entering an arbitrary specification is not advised.
“What should be done when only one specification exists?” The (only) specification you have should be used, and the other specification should be left out of consideration or treated as missing. In these cases, use only Zupper or Zlower.
Cpk can and should be calculated when only one specification exists, provided only the remaining valid specification is used. As the example demonstrates, the missing specification should remain missing and not be artificially inserted into the calculation.