As a subject matter expert in quality control and statistical analysis, I can provide you with an explanation of how to interpret a Cpk value and its relationship to the Sigma level.

Cpk, or process capability index, is a statistical measure that indicates how capable a manufacturing process is. It is calculated using the standard deviation (σ) of the process and the specification limits (USL and LSL). A Cpk value of 1.67 is considered to be a good process capability, as it indicates that the process is not only meeting but also exceeding the specification limits.

To determine the number of Sigma levels for a given Cpk value, you can use the following relationship:

\[ \text{Cpk} = \frac{\text{Specification Limit} - \text{Mean}}{\text{Standard Deviation}} \]

However, to find the Sigma level from Cpk, you need to consider that a Cpk of 1.67 corresponds to a process that is within 1.67 standard deviations of the mean for either the upper or lower specification limit. Since Cpk is a measure of both sides of the process, you would typically look at the smaller of the two distances (to the USL or LSL) to determine the Sigma level.

For a Cpk of 1.67, the process is generally considered to be operating at a Sigma level of approximately 4.5 to 5. This is because a Cpk of 1.67 means that the process is capable of producing parts that are within 1.67 standard deviations of the mean, which is a very low defect rate.

To summarize, a Cpk of 1.67 corresponds to a Sigma level of approximately 4.5 to 5, indicating a high level of process capability and quality.

read more >>

FormulaTwo sided tableCpkSigma level% out of tolerance1.334.00.011.504.50.0011.675.00.00018 more rowsread more >>