Meaning
Validation process used to confirm the accuracy of a calculated process capability index. Performing a cpk verification ensures that the relationship between the process mean and the spread relative to specification limits is mathematically sound. This audit compares raw measurement data against the summary statistics provided in a production report.
It prevents the acceptance of overstated capability values that might otherwise mask high scrap rates.
Mean Accuracy
Measurement of the average value relative to the target center identifies systematic bias. The average must remain stable over the entire production run for the index to be valid.
Spread Calculation
Estimation of the standard deviation determines the width of the process distribution. Using six times the standard deviation defines the total spread of the process. If the estimated sigma value is too low, the resulting index appears artificially high.
Correcting the spread requires a larger sample size or a more frequent sampling interval to capture periodic variation.
Index Margin
Buffer between the calculated value and the minimum requirement protects the assembly line from part interference. A verified index of 1.33 suggests that the process is capable of producing parts within tolerance at a high yield. Falling below this threshold triggers a process review or a containment action.
The margin accounts for the natural variation seen between different production shifts and raw material lots.
