The Evolution of the Shewhart Cycle


If you work in a Lean-focused organization, you are probably familiar with the plan-do-check-act (PDCA) improvement cycle. PDCA is a four-step cycle that promoted discipline and structure to promote ongoing learning and continuous improvement. The cycle begins with the Plan step. This involves identifying a goal or purpose, formulating a theory, defining success metrics and putting a plan into action. These activities are followed by the Do step, in which the components of the plan are implemented, such as making a product.


Next is the Check step, where outcomes are monitored to test the validity of the plan for signs of progress and success, or problems and areas for improvement. The Act step closes the cycle, integrating the learning generated by the entire process, which can be used to adjust the goal, change methods, reformulate a theory altogether, or broaden the learning – improvement cycle from a small-scale experiment to a larger implementation Plan. These four steps can be repeated over and over as part of a never-ending cycle of continual learning and improvement.

The History of PDCA

The history of the PDCA cycle is a unique example of the evolution of a theory and its various interpretations and implementations. Dr. W. Edwards Deming is noted by most as the person who popularized the improvement cycle. The PDCA cycle was based on the Shewhart Cycle.


Walter A. Shewhart, a noted statistician working at Bell Labs to develop the national telephone system. He created a three part cycle in 1939. This three-part cycle was a systematic process for continuous improvement. Shewhart wrote:

These three steps must go in a circle instead of in a straight line, as shown . . . It may be helpful to think of the three steps in the mass production process as steps in the scientific method. In this sense, specification, production, and inspection correspond respectively to making a hypothesis, carrying out an experiment, and testing the hypothesis. The three steps constitute a dynamic scientific process of acquiring knowledge.
Shewhart 3 step.png

When Deming introduced the cycle to the Japanese Union of Scientists and Engineers (JUSE) in 1950, he modified the cycle. He added a fourth step and emphasized to the Japanese engineers the importance of ongoing interaction between design, production, sales, and research. This modified Shewhart Cycle was referred to as the Deming wheel by the Japanese.


Deming discussed the five steps of the cycle:

  1. Design the product

  2. Make it and test in lab and production line

  3. Launch the product

  4. Test it in service via market research and user interviews

  5. Redesign and improve the product given customer’s feedback

The Cycle Gets Modified

After Deming’s work in Japan, the Japanese modified the cycle and renamed it the Plan-Do-Check-Act (PDCA) cycle. The four-step process focused on creating and modifying process standards to prevent mistakes. Deming instilled in his students that everything must be perceived as a process. Most problems are due to the process, not human error. This PDCA cycles was a key tool for kaizen, or continuous improvement. And became popular from Masaaki Imai’s famous book Kaizen: The Key to Japan’s Competitive Success and the success of the Toyota Production System.


Dr. Ishikawa

Dr. Ishikawa, the creator of the fishbone diagram and leading authority on the Quality Movement, modified the cycle in 1985. He included goals and targets with the Plan step as well as ways to achieve the goals. Ishikawa added education and training to the Do step for improved implementation. Proper control of the process according to Ishikawa was continual revision of standards based on the voice of the customer (VOC). This was based on Deming’s initial teachings that everything is a process and the standards that support the process must continually be revised and improved.


Along with the additional variables, Ishikawa added key statistical tools to develop quality. Examples of typical Lean tools used in a PDCA cycle are check sheets, histograms, Pareto charts, fishbone diagrams, scatter diagrams, and other tools for statistical process controls.

Deming’s Edits

In 1986 Deming reintroduced the four-step Shewhart Cycle. As Deming promoted this model in his 4-day seminars, he warned denounced the PDCA Cycle. Deming based this on the English definition of “Check” means “to hold back


Deming never embraced the PDCA cycle. He warned that PDCA was not accurate. He felt that the English definition of “check” means to “hold back”. He felt PDCA was a corruption of the original model. Deming continually told his students that PDCA and his four–step cycle were only related through the scientific method, which is the foundation of the cycle and process of continuous improvement.

In 1993 Deming again modified the Shewhart Cycle. Deming named this modified cycle as The Shewhart Cycle for Learning and Improvement – the PDSA Cycle. Deming noted the third stage as “Study” rather than “Check”. He felt it was more important to study and learn based on the results of the action, to drive improvement. He felt “Check” only focused on evaluation and did not result in continuous improvement.



Many firms still use the term PDCA rather than PDSA, especially as this was adopted from the success of the Toyota Production System and Lean. In Japan, many firms use Plan, Do, See which Deming never understood the use of. No matter which cycle you adopt, the benefits to the organization are a culture of ongoing learning, experimentation, and continuous learning. Ensuring a culture of continuous improvement and learning is critical to stay ahead of competitors and continually wow customers.