Capability Analysis: How is it made?

Step 5 Calculate the Cp and Cpk

Cp and Cpk are the most commonly-used capability indices.

a. Calculate Cp.
The Cp index is the allowable spread divided by the actual spread. The allowable spread is the difference between the upper and lower specification limits. The actual spread is 6 times the standard deviation (3 times the standard deviation for each side of the grand average). Plus or minus 3 times the standard deviation contains 99.73% of the data and is commonly used to describe actual spread. Cp for the example is:

b. Calculate Cpk.
The Cpk is simple to calculate. First, choose the smaller of the two Z values calculated earlier in Step 4. Call this the "minimum Z," or Z_min. The values found for Z_USLand Z_LSL are 6.11 and 2.78. Thus, Z_min�is 2.78. To find the Cpk, divide Z_min by 3. So the Cpk capability index for the example is:

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