Like many Japanese terms surrounding continuous improvement, there are several slight variations of translations of these three terms. In general, muda is the most commonly used of this group of terms. In practice, it has come to mean ‘waste’. Muda really means wasteful activity. Mura means the waste of inconsistency or unevenness. Muri is the waste of strain or unreasonableness.
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There are many different ways people categorize waste, for example, the sevenwastes, or memory devices such as ‘CLOSED MITT’. This focus on wasteful activities is understandable, of course, because the concept of pulling out wasteful activities is one of the foundations of continuous improvement.
Waste reduction is one of the central principles of continuous improvement.
Finding and identifying waste is embedded in any effort to make processes better. In fact, most of the Lean tools rely on waste reduction to be used effectively. Whether you are working on kanban, SMED, or flow, you need a thorough understanding of waste.
Mura and muri are particulary important concepts, though, as they are the sources of much waste. Uneveness in production forces teams to create processes to deal with the spikes in demand, and has teams sitting idle in slow periods. Unreasonableness, whether in pace, strain, or the types of tasks a person is asked to do all reduce productivity over time, and can increase turnover.
Mura and muri are particularly bad, as they are not commonly addressed as sources of waste. Most people can see scrap in a bin, or bad parts, common forms of muda, as wasteful. Fewer people can conceptualize how daily shifts in demand, or speeding up processes to hit deadlines drive waste into an organization.