Yes, Cpk can be negative.
A negative Cpk value indicates that the process mean is outside the specification limits. This means the process is producing output that is, on average, not within the acceptable range.
Here's a breakdown of what Cpk is and what different values mean:
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Cpk (Process Capability Index): Cpk is a statistical measure of how close a process is to its target and how consistent it is. It compares the process output to the specification limits.
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Specification Limits: These are the upper and lower limits that define the acceptable range for a product or process characteristic.
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Process Mean: This is the average value of the output produced by the process.
Interpreting Cpk Values:
| Cpk Value | Interpretation |
|---|---|
| Cpk > 1.33 | The process is capable, and the process mean is well-centered. |
| 1 < Cpk ≤ 1.33 | The process is capable, but improvements may be beneficial. |
| 0 < Cpk ≤ 1 | The process is not capable; a portion of the output falls outside specifications. |
| Cpk ≤ 0 | The process mean is outside the specification limits; the process is not capable. |
Why a Negative Cpk is a Problem:
A negative Cpk signals a serious problem. It means that the average output of your process is consistently outside the acceptable range defined by the specification limits. This results in a high percentage of defective products.
Example:
Imagine a process that is supposed to produce parts with a length between 9.5 cm and 10.5 cm. If the average length of the parts being produced is 11 cm, the Cpk would be negative, indicating that the process is not capable of meeting the required specifications.
In summary, a negative Cpk is a clear indication that the process needs immediate attention to bring its mean within the specified limits and improve overall capability.
