Mistake proofing is an effort to stop defects at the source. The prime objective is to prevent defects from occurring in the first place, but if they do occur, to stop their progression through the process. By stopping a defect at its source, its cost impact is minimized.
The further the defect progresses through a process, the more waste occurs. The more waste that occurs, the higher the cost impact. As a result, the best place to stop a defect is in the design of the process, product, or service. Once the process is in place, waste starts to be generated as a process output along with the product or service.
The first step in mistake proofing is to determine the kind of error, or errors, that caused the defect. In a Six Sigma project, this is what the Define, Measure, and Analyze phases have been isolating on a project level. As part of the Improve Phase, the problem process will be re-engineered. Part of this re-engineering will be mistake proofing the process steps.
There are general classifications of errors that lead to defects. Different organizations may have somewhat different categories.
Concentration: Lack of concentration, breaks in concentration, interruptions
Knowledge: Lack of training or experience
Judgment: Prejudice, expectation
Mistakes: Forgetting, accidents
Speed: Working too fast, working too slow
Standards: The absence of standardized work, absence of performance standards
Independence: Deciding to ignore rules or standards, freelancing
Intentional: Deliberate mistakes, sabotage
Incidental: Equipment failures, environment, surprises
Unknown: These will usually find their way into one of the above categories after analysis.
There are several approaches to mistake proofing. Each approach addresses at least one of the above error categories. The following are some of the more common strategies.
In manufacturing, one of the most common approaches is the use of fail-safe devices. These devices prevent the operator or machine from creating a defect. An example would be the use of a slipping-type torque wrench to prevent over tightening.
The magnification of the senses is another mistake proofing method. Examples would be optical magnification to improve vision and closed circuit video to see where it is not otherwise possible to see (distance, safety, etc.). Also used are pictures instead of numbers (LED bar charts instead or a numerical display on a meter) and multiple signals (audible and visual alarms used together).
The elimination of error-prone steps in a process is another method of mistake proofing. This may require designing a new process or the use of automation. An example of this is the use of ambient-light sensors to turn outside lighting on or off.
Facilitation of the work process will also aid in mistake proofing. This is changing the process steps so that they are easier to do, or easier to do right. 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.
Devices that detect an incorrect action or part can be used to mistake proof a process. Examples would include a weld counter to ensure the correct number of welds or a software modification that will not allow incorrect entries.
There are as many mistake-proofing strategies as there are mistakes. It requires communication and cooperation between the operators, the process owners, and the engineering staff to successfully execute. In many businesses these functions are silo’ed and do not work together well. This is why progressive companies are putting together production teams for both products and services. These teams are made up of dedicated operators, engineers, and managers all working in the same process. They all have ownership of the process, and as a result, communication and cooperation are easier to maintain.