ABSTRACT
It was Dr. Shewhart who first suggested the use of control charts. In the late 1920s, Dr. Shewhart suggested that every process exhibits some degree of variation. Since no two things can be produced exactly alike, variation is natural and should be expected. However, Dr. Shewhart discovered that there were two types of variation, chance cause variation and assignable cause variation. Chance cause variation is variation that is inherent in the process. It is random in nature and cannot be controlled. Any process that operates with only chance cause variation is said to be in a state of statistical control. Once a process is in statistical control, adjustments can be made to minimize the random variation, which will improve the process. Assignable cause variation is variation that is controlled by some outside influence or special cause, such as change in material, change in operator, change in tool setting, tool wear, or other phenomena. Any process that operates with assignable cause variation is said to be out-ofcontrol. By using Dr. Shewhart’s control charts, outside influence can usually be identified and controlled.
