The upper specification limit (USL) represents how far above the target your process can be.A specification for the width of a component, for example, might be 27 mm +/- 0.02 mm.

Specifications are defined in terms of nominal tolerances or ranges, using '+/-' to specify what the range is. Product specifications define the requirements a product must meet for it to be accepted by the customer.Just to confuse things, Cp is also sometimes called the Cp Index. Cp, or C p, stands for Process Capability.

Cpk, or C pk, stands for Process Capability Index.Of course, if it is a unilateral tolerance, then we can predict Sigma levels from Cpk numbers. Hence, Total PPM = 2700 => Sigma_Bench = 2.78.Ĭp * 3 gives you sigma level on one side (Z_LSL and Z_USL).Ĭpk cannot be directly translated into sigma level for bilateral tolerances as it only looks at the worst side. Let's say LSL = 11.5, USL = 18, Xbar = 15, S = 1.Ĭpk = min = min = 1.0Ĭonservative estimate for Sigma_Bench based on Cpk = 1.0 (assume both sides have PPM = 1350 - centered process!). However, assuming the worst case defects occurs on both sides, it would be possible to estimate a conservative estimate of Sigma level if we so desire. So, it would not be directly possible to translate Cpk numbers to Sigma Levels. So, if a process had 10,000 PPM defects to the left of LSL and 20,000 PPM defects to the right of USL, then Cpk will look at both of them and then compute the process capability number related to 20,000.Īs pointed out earlier, Sigma Level (Bench) looks at defects on both sides and adds them up. Which means, we are looking at the defects that are greater than USL on one side and the defects that are less than the LSL on the other side and only picking the worst case (A smaller process capability number relates to larger number of defects). When, we look at the formula, we see that we compute the minimum. Where, LSL and USL are the Lower and Upper Specification Limits (as determined from the customer), Xbar is the average of the process data, and S is the sample standard deviation. The traditional formula for Cpk (for a normally distributed process data) is given by:Ĭpk = min Here are some additional points regarding that topic.Ĭpk (unlike Cp) includes both variation and shift to calculate the process capability.

You have rightly pointed out the Cpk cannot be translated directly into Sigma levels.

How to calculate cpk formula