Process capability index (CpK) is one of the most commonly used statistical parameters for measuring and improving the processes as far as statistical process control (SPC) is involved. It is the process performance metric often used in all of the three stages of process validation (i.e., process design, process performance qualification, and continued process verification). As a rule of thumb, the default limit of no less than 1.33 is most widely used. In Parenteral Drug Association (PDA) Technical Report No. 60 – Process Validation: A Lifecycle Approach and International Society of Pharmaceutical Engineers (ISPE), the CpK as well as PpK with acceptance limits 1.0 and 1.33 are thoroughly discussed. PpK (process performance index) is another metric; it is calculated using the same formula as CpK with different standard deviations. This paper introduces the theory of sample CpK distribution with supporting simulation test results. The readers will have an insightful view on how sample CpKs are distributed and will understand why a sample CpK is sometimes much less than the true CpK or PpK. The author believes that the readers may have experienced some failure CpK results (i.e., less than 1.33); this paper will provide the answer to this particular failure.