The Significance of Source Inspections

Excerpted from Shingo, Shigeo, 1985, "Zero Quality Control: Source Inspection and the Poka-yoke System", Productivity Press, Portland, Orgeon, pp82-85.

Many people maintain that it is impossible to eliminate defects from any task performed by humans. This view stems from the failure to make a clear separation between errors and defects. Defects arise because errors are made; the two have a cause-and-effect relationship.

I claim that it is impossible to eliminate all errors from any task performed by humans. Indeed, inadvertent errors are both possible and inevitable. Yet errors will not turn into defects if feedback and action take place at the error stage. In this way, I am advocating the elimination of defects by clearly distinguishing between errors and defects, ie, between causes and effects. This is the principal feature of source inspections.

The problem can be visualized in the following way. Management systems in the past have carried out control or management in large cycles:

In source inspections, however, control or management is carried out in small cycles:

Zero defects are achieved because errors do not turn into defects, and management cycles are extremely rapid.

In general, we can imagine five situations in which defects occur:

  1. Cases in which either inappropriate standard work processes or inappropriate standard operating procedures are established at the planning stage. An example of this might be the setting of unsuitable heat-treatment temperatures. Since all products become defective in this sort of situation, real operations can not begin, of course, until these conditions are corrected.
  2. Cases in which actual operations show excessive variation even though standard methods are appropriate. An example might be the occurrence of occasional defects owing to excessive play in machine bearings. Here, too, operations can begin after proper maintenance has been performed.
  3. In cases where sections of raw material are damaged or material thicknesses fluctuate excessively, thorough inspections must be carried out when such materials are received.
  4. In cases where friction in machine bearings results in excessive play or worn out tools throw off measurements, overall tool management and maintenance need to be carried out.
  5. Some defects clearly occur in cases of inadvertent errors by workers or machines, eg., when chips clog parts. Such events are unpredictable and occur randomly, which makes them difficult for sampling inspections to capture. Here 100 percent inspections are indispensable.

The various situations described above recall something I have already said. The reduction in the defect rate at Arakawa Auto Body from 3.5 percent to 0.01 percent in the space of two years resulted from the adoption of source inspections, self-checks, successive checks, and poka-yoke devices. This fact proves, does it not, that the majority of defects are of the inadvertent error type 5?

Most of the remaining 0.01 percent of defects are those involving dirt, scratches, or other things, that are difficult to eliminate, The above methods completely do away with mismatched assemblies, missing parts, and similar defects.

Thus the most effective strategies for reaching zero defects are using source inspections to move through management cycles at the level of causes, and using source inspections in combination with 100 percent inspections and poka-yoke devices to speed up feedback and action.