Informative Inspection and Daily Work
Shigeo Shingo described three kinds of inspection methods in Chapters 4 and 5, Zero Quality Control: Source Inspection and the Poka-yoke System, Productivity Press, Stamford CT 1986.
He distinguished (1) judgment inspections; (2) informative inspections; (3) source inspections.
Judgment inspections sort good product --product meeting specification--from defective product to prevent shipping products with defects to customers. In service applications, a judgment inspection will only enable service recovery after you’ve delivered a service with defects.
Informative inspections represent a major innovation over judgment inspections: discovering defects is the first step in a feedback cycle to act on the cause system to reduce future production of defects.
Source inspections represent a further innovation: now you act to mitigate or eliminate the causes of defects, which are process errors. If you can contain the errors that cause defects, you can radically reduce or eliminate defects for many future production cycles.
I reproduced Figure 4.3 from p. 53 of Zero Quality Control at the top of this post. This figure summarizes informative inspections and source inspections:
- Errors are the cause of defects in product or service.
- If we can catch and correct errors before we make the product or service that would have defects caused by such errors, then there is no resulting defect. This is the short route to improvement and represents source inspection.
- Defects kick off the long route of informative inspection: defects provoke feedback and study that lead to action on the cause system.
Checklists are one tool to help us catch errors, to mistake-proof the process. The term 'Mistake-proof'--poka-yoke in Japanese --is Shingo’s invention. In chapter 7, Shingo provides 112 mistake-proofing examples in fabrication and assembly; these mechanical examples are great provocations to thinking about ways to improve service quality.
Connection to Daily Huddles and Daily PDSA cycles
Shingo’s picture uses different words but captures the steps of the Plan-Do-Study-Act cycle in both the short route and the long route. The cause system (process) is the Plan. When you Do the work, you may get errors (short route) or defects (long route). Check and Feedback corresponds to Study and action to Act.
Daily huddles that give people a few minutes to reflect on yesterday’s performance and act to make today’s work better are one way to build both source inspection and informative inspection into daily routine. Circumstances of yesterday’s errors and defects should be fresh in the minds of team members for a limited number of cases. Shingo’s challenge to us: reduce the time lags in source and informative inspection to drive continuous improvement. Don’t be satisfied with informative inspection; strive to achieve source inspection which will always be faster and more effective than informative inspection.
Statistical Quality Control
Statistical quality control—the use of control charts to characterize how well a production process meets requirements, identify problems, and support improvement— is one type of informative inspection. If you know something about statistical quality control and the history of quality improvement in Japan from 1950 to 1980, Shingo’s eight page forward offers a provocative view of the proper role of statistical methods in improvement.