Six Principles of Mistake Proofing

Some may say that it’s impossible to eliminate mistakes. And they are right; it is, most of the time. But, to be honest, they are missing the point.

Poka yoke, or mistake proofing, describes any behavior changing constraint that is built into a process to prevent an incorrect operation or act occurring.  The three aims of mistake proofing are:

• To reduce the risk of mistakes or errors arising.
• To minimize the effort required to perform activities.
• To detect errors prior to them impacting on people, materials, or equipment.

Ideally, poka-yoke ensures that proper conditions exist before actually executing a process step, preventing defects from occurring in the first place. Where this is not possible, poka-yoke performs a detective function, eliminating defects in the process as early as possible.

This can be achieved by following the 6 principles or methods of mistake proofing. These are listed in order of priority in fundamentally addressing mistakes:

1. Elimination seeks to eliminate an error-prone process step by redesigning the product or process so that the task or part is no longer necessary. This may require redesigning a new process or product simplification or part consolidation that avoids a part defect or assembly error in the first place.

Example:  An example of elimination is the use of ambient-light sensors to turn outside lighting on and off.

2. Prevention modifies the product or process so that it is impossible to make a mistake or that a mistake becomes a defect. This includes Limit switches to assure that a part is correctly placed or fixtured before process is performed; part features that only allow assembly the correct way, unique connectors to avoid misconnecting wire harnesses or cables, part symmetry that avoids incorrect insertion.

Example: An example would be a camera that will not function when there is not enough light to take a picture. Also some clothes dryers shut down when they detect an overheating situation.

3. Replacement substitutes a more reliable process to improve repeatability. This includes use of robotics or automation that prevents a manual assembly error.

Example: An example would be the implementation of an automatic dispenser to insure the correct amount of adhesive is applied during an assembly process or the coin dispenser in food stores preventing that customers are getting short changed.

4. Facilitation is the most used principle and employs techniques and combining steps to make a process step easier to perform or less error-prone. This includes visual controls including color coding, marking or labeling parts to facilitate correct assembly; checklists that list all tasks that need to be performed; exaggerated asymmetry to facilitate correct orientation of parts.

Example: An example would be to color code parts that are similar in shape. This would make it easier to identify the correct part for assembly. Another example would be the use of a slipping-type torque wrench to prevent over tightening. When gas stations introduced unleaded gasoline, the nozzle on the leaded pump was designed to be too big to fit into an unleaded tank, thereby preventing mistakes. Electrical outlets have been mistake proofed to assure proper polarity. It is impossible to put a plug in an outlet incorrectly.

5. Detection involves identifying a mistake before further processing occurs so that the operator can quickly correct the defect. This includes sensors in the production process to identify when parts are incorrectly assembled; scales to measure and control the weight of a package; built-in self-test capabilities in products.

Example: Examples would include a weld counter to ensure the correct number of welds or a software modification that will not allow incorrect entries. Also warning device, using sound and light, like the seat belt buzzers, can be used to predict when something is about to go wrong.

6. Mitigation seeks to minimize the effects or the mistake. This includes mechanisms that reduce the impact of a error and defect; products designed with low-cost, simple rework procedures when an error is discovered; extra design margin or redundancy in products to compensate for the effects of errors.

Example: An example would be a smoke or heat detector detecting a hazardous situation. Also fuses to prevent overloading circuits resulting from shorts are mitigation techniques.

Ideally, mistake-proofing should be considered during the development of a new product to maximize opportunities to mistake-proof through design of the product and the process (elimination, replacement, prevention and facilitation). Once the product is designed and the process is selected, mistake proofing opportunities are more limited (prevention, facilitation, detection and mitigation).

Eradicating human errors is crucial to any business. Errors cost money and impact customer satisfaction. By introducing simple measures to trap and stop errors organizations can not only save costs but also become more efficient.