I did an online search recently on the ideas of “Quality Control” and “Quality Assurance” and I discovered that based on the information found online it seems there is not much difference between the practice of Quality Control and that of Quality Assurance. It seems to me that the terms might be used interchangeably in the Lean Logos today and therefore in the Lean Six Sigma practice. I am writing this blog because the idea that Quality Control and Quality Assurance practices are the same thing is false. The actual work involved in the practice of Quality Control is quite different from the actual work involved in the practice of Quality Assurance. Who does this work is also different and both practices rely on clearly defined process standards. I would like to unpack this idea for you today.

When I read the writings of Shigeo Shingo¹ and John R. Costanza², I found that there is a distinct difference in the work content for the practice of Quality Control and that of Quality Assurance. The work involved is not the same. I also found that each of these esteemed LEAN practitioners described the work content for the two practices yet referred to them by different names. What Costanza refers to as “Quality Control”, Shingo calls “Successive Checks”. What Costanza refers to as “Quality Assurance”, the monitoring of process performance using statistical process control, Shingo calls “Quality Control”. Yet the work involved is quite different. I think this history is important.

As you know, Shingo writes from his experience in the manufacturing environment. Costanza writes from his experience in the manufacturing environment that has a “mixed-model-flow”². That is to say, that not every unit of work goes through the same sequence of steps in the facility. Yet, the ability to standardize process, to practice quality control in the process, and practice the monitoring of the process using statistical techniques is applicable. This is important for two reasons. Firstly, the technical competencies for the practice of Quality Control is different from that of Quality Assurance. The tools are different. The skills are different. The intentions are different. The second reason this is important is because LEAN has expanded beyond manufacturing. The need for process standards, quality control, and quality assurance exists in the service sector and transactional environments.

So what are the differences between Quality Control and Quality Assurance?

Quality Control happens at the point-of-work within the process flow in the operation. It relies on what Shingo calls “successive checks”. That is to say that the output from step-1 is evaluated for quality at step-2 by the person doing the work at step-2. This “successive check” happens within the flow of the process; it is not outsourced to a different department for formal inspection and quality check. This is how quality gets built into the process in the actual doing of the work. When the unit fails to meet the successive quality check, that is when the Andon gets triggered (1, p.32-35, p.44-48). Now, in his writing Costanza expands on Shingo’s idea of “successive checks”. Where Shingo left the interpretation of what “good” looks like up to the person at Step-2, Costanza clarifies that “Total Quality Control Criteria” (TQC) can be defined for the output at step-1 and then those specific TQC criteria is what constitutes the in-process-control check (IPC) that gets done at the point of work before more value is added to the unit (2, p.155, p.180). This is precisely how quality gets built into the doing of the work at the point of work within the operation.

Conceptually, if that is what makes up Quality Control, then what is the purpose of “Quality Assurance”? Shingo explains in his writing how his understanding of the purpose of “inspection” evolved with the science of statistical sampling and monitoring using statistical process control (SPC). The statistical sampling and monitoring of the process is needed for doing just that: monitoring the process performance. However, the use of control charts and statistical sampling cannot prevent the defect from moving downstream¹. At best it can identify the existence of the defect after the fact (1, p.32-35, p.44-48). Costanza elaborates on this further in his writing. The purpose of random sampling and using SPC to monitor process performance is to keep track of process performance, identify areas where improvement might be needed, and adjust the takt-time for steps within the flow that might need to operate at a different pace until the defect problem is solved (2, p.54-58).

From this narrative, you can see the very nature of the practice of Quality Control is quite different from the practice of Quality Assurance. Both practices are needed. Both practices are related to the ability to define process standards for the work that gets done at each step and then build the quality into the process and then monitor the process performance. To be sure, these terms refer to technical practices and are not necessarily the names of organization departments (though some organizations might have departments by these names).

The defining of process standards, the practice of quality control at the point of work, and the practice of quality assurance are each practical and tactical and technical. Knowledge, skill, and competence are key. To learn more about the tools applied in each area, refer to Chapter-1 from my book The Art & Science of Applied Lean for Operations ^TM: Lean Sigma Practice Beyond Certification and Chapter-3 and Appendix-C from my second book Applied Lean for Operations: Process Design and Development Methodology ^TM. The Anatomy & Physiology of Process.

References:

1. Shingo, Shigeo. Fundamentals Principles of Lean Manufacturing. WA: Enna Products Corporation and PCS Inc. English translation 2009
2. Costanza, John R. The Quantum Leap. In Speed to Market. CO: John Costanza Institute of Technology, Inc. 1996
3. Constantinescu, Azizeh E. The Art & Science of Applied Lean for Operations ^TM: Lean Sigma Practice Beyond Certification. The Oakley Press. 2017
4. Constantinescu, Azizeh E. Applied Lean for Operations: Process Design and Development ^TM. The Anatomy & Physiology of Process. The Oakley Press. 2020