The Shingo system developed in parallel and in many ways in conflict with the statistically based approach to quality control. This system, or philosophy of production is named after the codeveloper of the Toyota just-in-time system, Shigeo Shingo. Two aspects of the Shingo system in particular have received great attention. One is how to accomplish drastic cuts in equipment setup times by single-minute exchange of die (SMED) procedures. The other, the focus of this article, is the use of source inspection and the poka-yoke system to achieve zero defects.

Shingo has argued that SQC methods do not prevent defects. Although they provide information to tell us probabilistically when a defect will occur, they are after the fact. The way to prevent defects from coming out at the end of a process is to introduce controls within the process. Central Shingo's approach is the difference between errors and defects. Defects arise because people make errors. Even though errors are inevitable, defects can be prevented if feedback leading to corrective action takes pace immediately after the errors are made. Such feedback and action require inspection, which should be done on a 100 percent of the items produced. This inspection can be one of three types: successive check, self-check, and source inspection. Successive check inspection is performed by the next person in the process or by an objective evaluator such as a group leader. Information on defects is immediate feedback for the worker who produced the product, who then makes the repair. Self-check is done by the individual worker and is appropriate by itself on all but items that require sensory judgment (such as existence or severity of scratches, or correct matching of shades of paint). These require successive checks. Source inspection is also performed by the individual worker, except instead of checking for defects, the worker checks for the errors that will cause defects. This prevents the defects from ever occurring and, hence, require rework. All three types of inspection rely on controls consisting of fail-safe procedures or devices (called poka-yoke). Poka-yoke includes such things as checklists or special tooling that (1) prevents the worker from making an error that leads to a defect before starting a process or (2) gives rapid feedback of abnormalities in the process to the worker in time to correct them.

There is a wide variety of poka-yokes, ranging from kitting parts from a bin (to ensure that the right number of parts are used in assembly) to sophisticated detection and electronic signally devices. There is a good deal more to say about the work of Shingo. Blasting industry's preoccupation with control charts, Shingo states they are nothing but a mirror reflecting current conditions. When a chemical plant QC manager proudly stated that it had 200 charts in a plant of 150 people, Shingo asked him if "they had a control chart for control charts." A. Robinson, Modern Approaches to Manufacturing Improvement: The Shingo System (Cambridge, MA: Productivity Press, 1990), p. 234. In addition to his insights into the quality area, his work on SMED is a must read for manufacturing executives.