In probability theory and statistics there are many distribution functions that fit well to describe observations of a physical phenomenon. Distribution functions come with different averages, dispersions (spreads), and shapes. Binomial, Poisson, exponential, hypergeometric, chi-squared, student’s t, and so forth distributions and their applications can be found in many probability and statistics books. The most applicable and important frequency distribution that fits observed and variable data in high-volume manufacturing and that helps us generate process control charts by sampling our product is the normal distribution. There are two main reasons why we focus on the normal distribution for process control charts.