Unlike Cp, Cpk makes no assumption that the process mean is centered between the specification limits. The formula for the calculation of Cpk is Cpk = min(USL – μ, μ – LSL) / (3σ) where USL and LSL are the upper and lower specification limits, respectively.Click to see full answer. Moreover, what is the formula for CP and CPK? Calculating Cpk using a Z Value If you have a Z value, the equation is very easy; Cpk can be determined by dividing the Z score by three. A z score is the same as a standard score; the number of standard deviations above the mean. Z = x – mean of the population / standard deviation. what is CP CPK value? Cp and Cpk. Cp and Cpk, commonly referred to as process capability indices, are used to define the ability of a process to produce a product that meets requirements. In other words, they define what is expected from an item for it to be usable. Similarly, you may ask, what does a Cpk of 1.33 mean? Process capability index (Cpk) is a statistical tool, to measure the ability of a process to produce output within customer’s specification limits. Cpk = or >1.33 indicates that the process is capable and meets specification limits.What is difference between CP & CPK?Cp is the simplest indicator of a process capability while Cpk gives a better picture. 2. Cp is also known as the “process potential index” while Cpk is known as “process capability index” or “process performance index”.