Poka Yoke (Mistake Proofing)

1. Introduction

Poka yoke is a Japanese term that means "mistake proofing". A poka yoke is any mechanism in a Lean Manufacturing process that helps an equipment operator to avoid mistakes. It also means taking steps to ensure errors or abnormalities can't occur, which involves using checklists, quality checks, part or tool design, machine modification, setting tolerance limits and so forth. Its purpose is to eliminate product defects by preventing, correcting, or drawing attention to human errors as they occur. The concept was formalized, and the term adopted, by Shigeo Shingo as part of the Toyota Production System.

2. Poka Yoke Types

There are three types of Poka yoke:

• Contact type:
  The use of shape, dimensions or other physical properties to detect the contact or non-contact of a particular feature.
  E.g. things only fit one way.

• Constant number type:
  If a fixed number of actions or movements are not made an error sign is triggered.
  E.g. complete data entry fields when taking an order.

• Performance sequence type ensures steps are performed in the right order.
  E.g. the use of a checklist for pre-flight checks or completing forms in a logical way.

3. Types of Error

Poka yoke can be used wherever something can go wrong or an error can be made. It is a technique, a tool that can be applied to any type of process in manufacturing or the service industry. There are many types of error:

1. Processing error
   Process operation missed or not performed per the standard operating procedure.

2. Setup error
   Using the wrong tooling or setting machine adjustments incorrectly.

3. Missing part
   Not all parts included in the assembly, welding, or other processes.

4. Improper part/item
   Wrong part used in the process.

5. Operations error
   Carrying out an operation incorrectly; having the incorrect version of the specification.

6. Measurement error
   Errors in machine adjustment, test measurement or dimensions of a part coming in from a suppler

4. Implementation

1. Identify the operation or process that needs to be mistake proofed (target areas where there are high numbers of errors or where even single errors are very costly).
2. Use the 5 Whys or cause and effect analysis to get to the root of the problem.
3. Decide whether to use a shut-out (preventing an error being made) or attention type (highlighting that an error has been made.) method (there may be technical or financial reasons why you have to go for the latter) to tackle the problem.
4. Decide whether a contact, constant number or sequence method is best (this will hinge on the nature and purpose of the activities).
5. Design an appropriate poka yoke.
6. Test it to see if it works (try to avoid large expense before you have completed this step - use mock ups or make-do's).
7. Once you have a working method then ensure you have the right tools/checklists/software, etc for it to work consistently and correctly.
8. Train everyone to use it.
9. After it has been in operation for a while (the time period will depend on the frequency of the activity) review performance to ensure errors have been eliminated.

Take whatever steps are needed to improve on what you have done.

5. Application in Company

We can apply it to our company in certain tasks, such as software development process, and management & maintenance tasks on the intranet.

When a developer starts working on a new project, maybe he will encounter the same problem sever times (or spend much time to handle it). So he can use this technical to solve and avoid this kind of similar mistake next time. First he needs to identify the process causing much number of errors. Then get the root of the problem, and decide whether to use a shut-out or attention type method. Test the poka yoke you worked out whether it works correctly and properly, and document the right checklists. Finally, you should tell the other related people how to avoid the problem.