While both these approaches will measure certain aspects of a process, neither reveals what to improve, how to improve it or considers the amount of variation that occurs within the data or the process the data represent. Unless the level of variation is measured there is no way of determining if improvement has occurred . In 1924, Dr. Walter Shewhart, a noted scientist employed by Western Electric, devised a framework for the first application of statistical control through the development of control charts also known as process behavior charts. His method of analysis is fundamental to the projection of current process characteristics to future process performance. Shewhart established that there are two types of variation , routine and exceptional .
Routine variation is unavoidable and is inherent to a process.
A process that displays routine variation reflects consistency. It is a predictable process. Consequently current process characteristics can be a guide to future process performance. Exceptional variation, however, is inconsistent, changing over time. Processes that display exceptional variation are unpredictable.