Shewharts

Shewharts refers to the body of control-chart methods and related quality control concepts developed by Walter A. Shewhart, a physicist and statistician at Bell Telephone Laboratories. In the 1920s and 1930s he articulated a framework for monitoring manufacturing processes and distinguishing between common-cause variation and assignable (special) causes. His work laid the foundation for statistical process control and fundamentally shaped modern quality management.

The core of Shewharts contribution is the control chart, a statistical tool used to track process measurements over time and determine whether a process remains in a state of statistical control. The classical family of Shewhart charts includes the X-bar chart (monitoring process means) with accompanying R charts (monitoring dispersion) or S charts for standard deviation, as well as the Individuals chart for single measurements and the Moving Range chart to track variability between successive observations. Control limits, typically set around three standard deviations, serve as signals that the process may be affected by an assignable cause.

Impact and legacy: Shewharts methods became the foundation of statistical process control (SPC) and influenced later quality pioneers such as W. Edwards Deming. They underpin many modern quality improvement practices and the planning of systematic process evaluation. The PDCA (Plan-Do-Check-Act) cycle, which Shewhart helped popularize in its early form, also contributes to continuous improvement efforts inspired by his work.

In summary, Shewharts denotes the group of control-chart techniques and the broader SPC approach introduced by Walter A. Shewhart, which remains central to quality control in manufacturing and service processes.