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How to avoid human mistakes in production

by Renaud Anjoran

As Shigeo Shingo wrote, “humans are animals that make mistakes”. But how often do we make mistakes?

For example, trained military technicians make mistakes 20% of the time in simulated emergency situations. Is it scary?

You can see many more such examples by clicking here.

So the solution is to get rid of human operators and to automate the whole process, right? I do not agree.

And that leaves us with “animals that make mistakes” at the heart of production. The Lean movement, following Toyota’s example, offers effective solutions to reduce human errors.

1. The general philosophy

As Jon Miller points out, there must be a non-negotiable acceptance by everyone of the following:

  • We do not accept bad work
  • We do not do bad work
  • We do not pass on bad work

In Chinese factories I regularly see employees creating defects, or working on parts that are faulty, and not doing anything about it — even though they know there is a problem!

I have found that, if the pay structure does not create the right incentives, this general philosophy will be ignored. If workers are paid by the piece and if there is no way to trace bad products back to a certain person, quality is the last priority.

But if the team gets a bonus based on the number of GOOD pieces produced, the mentality changes quickly.

2. Mistake-proofing

Have you ever managed to insert the battery of your cell phone upside down? Or to spill gas at a station, while the pump is in your car’s tank?

Probably not, because the manufacturers implemented mistake-proofing devices. Once one starts watching processes with these examples in mind, one can come up with many ideas.

You can see some examples in a factory setting by clicking here.

3. Self-inspection

Counting on quality inspectors to look for defects has a huge downside: problems might be found hours (or days) after they appear. In the meantime, hundreds of bad pieces might have been produced. And finding the root cause of these problems might be very difficult (the trail got cold).

So the best is to have operators self-inspect. However, does an operator have the objectivity necessary to check his own work? Probably not. Fortunately, solutions exist.

Here is what Michel Baudin advises, in his excellent book Lean Assembly:

  • For manual assembly, successive inspection is a viable alternative to self-inspection. Instead of trying to find fault with his or her own work, each assembler does it on the work done at the previous station. This usually provides enough psychological distance for it to be effective.
  • For mechanized or automated assembly, the work of putting components together is not done by the assembler but by a machine, and self-inspection is possible, because it is the machine that the assembler checks on.
  • The clearer the inspection criteria, the easier it is to implement self-inspection. The questions of whether all screws are in place, a plug gauge goes into a hole, or a green light on an instrument comes on leave no room for interpretation or subjectivity. The same is not true, for example, of inspecting a painted surface for scratches or runs.
  • What is most difficult for an assembler to do is design a self-inspection procedure. On the other hand, it is not nearly as challenging to go through a checklist designed by someone else.

Some simple tools also help a lot in making defects obvious to supervisors and quality engineers.


Renaud Anjoran is the founder of Sofeast Quality Control and helps importers to improve and secure their product quality in China. He writes advice for importers on the Quality Inspection blog. He lives full time in Shenzhen, China. You can contact him at This email address is being protected from spambots. You need JavaScript enabled to view it..

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