Shewhart Cycle

The Shewhart cycle, also known as the Plan-Do-Check-Act (PDCA) cycle, is an iterative four-step management method for the control and continuous improvement of processes and products. Developed by physicist and statistician Walter Shewhart at Bell Laboratories in the 1930s and later popularized by W. Edwards Deming, the cycle formalizes the scientific method as a repeating loop: Plan (establish objectives and processes), Do (implement the plan on a small scale), Check (study the results against expectations), and Act (standardize improvements or adjust the plan).

The Shewhart cycle is the methodological ancestor of modern iterative development in software engineering. Where Shewhart applied the cycle to manufacturing quality control, software engineers apply it to requirements uncertainty — but the epistemological structure is identical: hypothesis, experiment, measurement, adaptation. The cycle is not merely a project management technique; it is a formalization of how learning happens in environments where the correct specification cannot be known in advance.