Inspection does not reduce the amount of defects

As defects are inherent to production processes, the need for inspection rises in today’s manufacturing environment. Although inspection does not reduce the amount of defects, it is a common technique to prevent outflow of defects to the customer. The way this inspection is done is of huge importance.

Shigeo Shingo (1986) identifies three different ways of inspection:

  1. Judgement inspection
  2. Informative inspection
  3. Point-of-Origin inspection

1. Judgement inspection:

Also called standard inspection. This is the most traditional, very expensive, not efficient but still most occurring in today’s manufacturing, method of inspection. As the word says it all, a judgement must be made to sort the defects out. Often in manufacturing terminology this is referred to as a “Quality Wall”. Judgement inspection takes place after the defect has occurred.

On the picture below the inspection point takes place at the end of the 4-step manufacturing process. This means all value added activities are already performed before the defective part is identified. If the defect originates for example in step 1, time and money are lost by further processing the already defective part in the subsequent steps. After the defect gets filtered out, it is checked and feedback is given to request action in the process step where the error is made. This way of standard inspection is slow in reactivity, inefficient and very costly.

Standard Inspection
Standard Inspection

Generally a 90% effectiveness is accepted for judgement inspection. This implies that if 10 000 items need to be inspected of which 1 000 are a defect, the first Quality Wall will miss 100 out of 1000. The second Quality Wall will miss 10 and the third Quality Wall will miss out 1. This proves that judgement inspection is not effective, nor efficient.

3-Step Quality Wall
3-Step Quality Wall

Judgement inspection supports in preventing outflow of defects to the customer, but does not actively contribute in reducing the defect rate. It adds an additional step to the production process and is performed not only on defective parts but also on good parts, which makes it even more ridiculous. Even when this inspection is automated, it is not value adding and therefore a waste of resources. By all means, this method of inspection should be minimized or even eliminated from your production process.

2. Informative inspection:

Informative inspection analyses data to control and prevent defects. Statistical Process Control (SPC) is a way of informative inspection that serves as a quality control method based on sampling and statistical methods. Key tools in SPC are control charts and Design of Experiments (DoE). No further details are provided here on SPC but the basic principle is that the statistics give feedback in order to alter the working methods to reduce the defect ratio.

Xbar-R chart
Xbar-R chart

Informative inspection takes place after the defect has occurred but supports in reducing the defects percentage. Therefore it is called the passive approach. Shingo (1986) argues that SPC is a tool to keep your defect ratio constant and not to reduce it. The technique requires you to take samples at sufficient intervals and performs an analysis on the results. As a result, SPC is less fast in taking corrective actions and less thorough in comparison with the next inspection method: Point-of-Origin inspection. Although SPC is not the most ideal way of inspection, we consider it as a strong set of tools which are expected to be know by each quality member in the manufacturing area.

3. Point-of-Origin inspection:

Point-of-Origin Inspection is what we want to encourage in each manufacturing process. If focuses on inspection as close as possible to where the defect occurs or would occur. All three methods below imply 100% inspection, the only difference lies in where the inspection takes place.

Self Inspection
Self Inspection
  • Self inspection:

Self inspection is, as the term explains, an inspection process by the operator that produces the product. The problem with self inspection is that the operator controls him or herself. He or she might then compromise with the quality level of the part that he or she has produced and might not judge the quality objectively. The solution in that case lies in successive inspection.


  • Successive inspection:

Successive inspection or “next-process-checks” are installed in the subsequent process step and not in an independent inspection step. As each process step requires incoming parts, successive-checks are installed to prevent flow-through to the next process. Each operation consists out of inspection and production. When a defect is detected, it is immediately fed back to the previous step where it can be corrected. This method of inspection will be even more efficient when poka-yoke devices are used.

Successive Inspection
Successive Inspection

For example when operator A produces a sub-assembly toy and passes it through to operator B who paints the toy. Operator B first inspects the parts. If it is not OK, she gives it back to operator A for rework. If this is combined with a poka-yoke, operator B could start by putting the part in a jig before being able to start the painting process. If the toy does not fit in the jig, it is detected as a defect and fed back to operator A.

This method though has some drawbacks according to Shingo (1986). First of all, when too many items need to be checked, some might be overlooked. A general rule applied is that the maximum number of characteristics to be checked for can be maximum 3. Secondly, the method does not work if the operator does not follow the feedback principle and no corrective action is taken. At Toyota companies, therefore the Stop-Call-Wait procedure is executed. The production line stops until corrective action is taken. This gives responsibility to the workers and puts high pressure on the management to act quickly. Thirdly, the method might get resistance because on the short term, internal defects will rise. This is off course a misunderstanding because the defects are now found earlier in the process instead of at the end. Finally, this method can be dangerous for inter-worker relationships as the control might be conceived as criticism. Good communication and explanation to the workers is therefore mandatory.

Safety related items can even undergo double-successive inspection. In this case not only the next operator, but the second next also inspects the parts on the same characteristic. For example in a safety relevant arc welding process, the welder first counts himself the amount of welds he has made. After the arc welding operation, the frame goes to spot welding, where the spot welder first needs to check the amount of arc welds performed before starting the spot welding process.

  • Self inspection revised:

Successive inspection has proven its efficiency, but it would still be better if there was no time lag between the occurrence of the defect and the detection by the next operator. It would be more efficient if immediate feedback was given to the actual operator, but as mentioned before, this operator might not judge objectively.

If this judgement could be performed objectively, the flaw of self inspection would be tackled. This is where poka-yoke devices come to the rescue. “Self-checks” uses poka-yoke devices to inspect own work. The operator performs his task and immediately a confirmation is given that the part is OK or NOT OK. This allows instant correction of the defect and contributes to even better quality performance. In particular cases, successive inspection would offer feedback too late and the root cause for the defect might have vanished already. Self-check systems prevent this from happening but still face the problem that the defect occurred. This is tackled in the last inspection method: Source inspection.

  • Source inspection:

    Source Inspection - locator pins
    Source Inspection – locator pins

Source inspection is most ideal. This inspection method takes place before the defect occurs. It is based on the principle to check the operating conditions that result in defects. Here, the poka-yoke device checks before the operation takes place if all conditions are met and so eliminates the cause of defect. An example is a locator pin. The factor that causes the defect (misalignment) is checked, not the defect itself.

Self-checks and successive-checks should be installed if source inspection is not possible.

Point-of-origin Inspection
Point-of-origin Inspection

The link with poka-yoke and each of the above methods of inspection is that poka-yoke devices can be installed in each method of inspection, reaching its highest efficiency in source inspection.

Source inspection, Self inspection and Successive inspection
Source inspection, Self inspection and Successive inspection

Zero Quality Control

Source inspection leads together with poka-yoke and immediate response to the concept of Zero Quality Control (ZQC). They support in reaching a zero defect target. Successive checks and self-checks result still at least in 1 defect before it is detected. Even though it could be repaired, it results in a form of waste. In source inspection no defects occur because the errors are detected before they turn into defects.

According to Shigeo Shingo (1986), a Zero Quality Control system is build on the following principles:

  1. Source inspection should be applied. It means that inspection should take place where the defects arise.
  2. 100% inspection is preferred above sampling inspection.
  3. The time between defect and corrective action should be minimized.
  4. Errors are inherent to human people. This should be recognized and the system should be build in that way that these human mistakes are prevented.

Try to take these insights in mind when designing or reviewing your production process.

Note: Multiple sources were used for summarizing the content on this page. You can find these sources on the following page: References.