369. Cpk
369.1. (Process Capability Index with Centering)
Minimum distance from mean to nearest spec limit divided by 3 standard deviations. Process capability considering centering and variation.
If process is centered:
If process mean is off center, the smaller side of the process-to-limit ratio determines :
If , process is not centered
| Value | Meaning | Quality Impact |
| Process not capable, output falls outside specs |
Many defects, poor performance |
|
| Process just meets specification limits |
Acceptable only if well centered |
|
| Process is capable, centered or nearly so |
Fewer defects, reliable quality |
|
| Excellent capability, Six Sigma level |
Near-perfect quality |
Example
Suppose a company manufactures metal rods, and the specification limits for the diameter of the rods are:
- Target: 10 mm
- Lower Specification Limit (LSL): 9.8 mm
- Upper Specification Limit (USL): 10.2 mm
The process has:
- A standard deviation of 0.05 mm.
- A process mean of 10.1 mm.
Step 1: Calculate the distance from the mean to the USL:
Step 2: Calculate the distance from the mean to the LSL:
Step 3: Determine :
Interpretation:
= 1.33 means the process spread (6 ) fits 1.33 times within the tolerance range (the distance between the Upper Specification Limit and Lower Specification Limit.
- = 1.00: Process mean is exactly at the midpoint of the specification limits, and the process variation fits exactly within these limits. 99.73% of the output will be within specifications, indicating a capable process (3 sigma process).
- : The higher the , the more capable and stable the process is, meaning it can consistently produce parts within tolerance with minimal risk of defects.
- : The process mean is off-center or the variation is wider than the specification limits, or both. A significant portion of the output may fall outside the specification limits.