Three terms often used together in the Toyota Production System (also called the three M's) that together describe wasteful operations that should be eliminated.
Any activity that consumes resources without creating value for the customer. Within this general category, it is useful to distinguish between type one muda, activities that cannot be eliminated immediately, and type two muda, activities that can be quickly eliminated using kaizen.
An example of type one muda is repair work after defective treatment in a paint booth, which is necessary to obtain a finished product acceptable to the customer.
Because manufacturers have been searching in vain for decades to find an adequate paint process for fine finishing, this type of muda is unlikely to be eliminated soon.
Examples of type two muda include multiple movements of products and supplies between steps in a production and assembly process. These steps can be quickly eliminated using a kaizen workshop by housing production equipment and operators in a smooth-running cell.
Irregularities in an operation, such as erratic scheduling caused not by end consumer demand but by the production system, or an irregular pace of work in an operation that causes operators to rush first and then wait. Irregularities can often be eliminated by managers if they level out and pay careful attention to the pace of work.
Overloading equipment or operators by making them work faster or harder for an extended period of time than equipment design and/or decent personnel management allows.
Muda, mura and muri together
A simple example shows how muda, mura and muri often occur in combination, so that the elimination of one often leads to the elimination of the other two as well.
Suppose a company needs to transport six tons of material to its customer. One option is to load all six tons onto one truck and make one trip. However, this would be muri because the truck (which is calculated to carry a maximum of three tons) would then be overloaded. This creates the risk of damage and breakdowns, which in turn would lead to muda and mura.
A second option is to make two trips, one with four tons and the other with two. This would be mura, as having the material arrive at the customer in uneven numbers would lead to congestion at the receiving dock, followed by not enough work. Moreover, this option would lead to muri, due to the truck still being overloaded on one of the trips, and also to muda; the irregular pace of work would be wasteful, due to the customer's employees having to wait.
A third option is to load two tons on the truck and make three trips. That might not be mura and muri but it would be muda, because the truck would be only partially full each trip.
The only way to eliminate muda, mura and muri is to load three tons on the truck (the specified capacity) and make two trips.