Lean tools

Muri and mura

In French: Muri et mura — surcharge et irrégularité

Muri is the overburden imposed on people and equipment; mura is the unevenness of the load over time. They are the two causes that produce the waste one is about to fight.

Muri: what exceeds what is sustainable

Muri is excessive strain: a station whose content exceeds the available time, a machine pushed beyond its rating, a team asked for a volume it can hold only by giving something up.

It never shows under its own name. It is read in strained gestures, painful postures, shortcuts taken on the operating procedures, hours that overrun, absenteeism and premature breakdowns. Those are the symptoms usually treated one by one, without going back to the load that produces them all.

Its particularity is to be costly long after it has disappeared: a machine mistreated for six months breaks down the following year, and a team subjected to an unsustainable pace loses skills that will take far longer to come back than the overburden lasted.

Mura: what varies for no reason

The same quantity of work presents itself in two ways: spread out, or concentrated. Mura refers to the second, and the gap between the two is enough to separate a calm organisation from one that is always running — for an identical annual volume.

An organisation sized for the peaks has unused means the rest of the time; sized for the average, it suffers delays at every peak. There is no third option as long as the unevenness itself is not treated.

Much unevenness is manufactured by the organisation rather than endured from the customer: launches grouped at the end of the month, approvals concentrated on Mondays, weekly rounds. Those are the easiest to correct, and the least often looked at because they seem natural.

Why start with them

The visible wastes are often the consequence of these two. A stock exists to absorb an unevenness; rework comes from work carried out in the urgency of a peak; a wait is the trough that follows the bump.

Attacking waste without treating its cause gives results that do not hold: the removed stock comes back as stock-outs, and the organisation concludes that the approach does not work for it.

The useful order is therefore to level first what can be levelled, then bring the load back to what is sustainable, and tackle the residual wastes only afterwards. It is less spectacular at the start, and it avoids having to start over.

The common mistakes

Taking unevenness for a customer datum. A large share comes from internal rules — launch calendar, order thresholds, processing days — and is corrected without asking anything of anyone outside.

Treating overburden with additional means only. Adding resources without reducing what causes it moves the threshold without changing the regime, and the overburden returns at the first disruption.

Confusing high load with overburden. An organisation can work a lot without excess; what defines muri is exceeding what is sustainable over time, not the level of activity.

On the shop floor — Testing laboratory

Turnaround times slipped every other day and the team was asking for an additional position. The record of arrivals showed that 70 % of the samples came in within a two-hour window, because all the collection rounds came back at the same time. Staggering two rounds removed the peak without changing the volume processed.

The other tools illustrated by this trade: healthcare and life sciences.

Frequently asked questions

What is the difference between muda, mura and muri?

Muda is the waste observed — what consumes without contributing anything. Mura and muri are two of its causes: the unevenness of the load and its excess. That distinction changes the order of actions, because removing a waste whose cause remains makes it reappear in another form, often less visible than the first.

How do you spot muri on the shop floor?

By looking at the compensations rather than the complaints: strained gestures, shortcuts on the operating procedures, constant mutual help to hold the pace, overruns that have become normal. Those signs come well before a difficulty is voiced, because overburden settles in gradually and everyone regards it as the usual way of working.

Does levelling mean making the customer wait?

Not necessarily, and it is the most frequent objection. A large share of unevenness comes from internal decisions that change nothing about the delivery date: launch order, batch sizes, processing days, administrative groupings. That share is treated first, at no cost to the customer, before considering anything that concerns them.

Should mura be treated before muri?

Most often yes, because part of the overburden exists only during the peaks: levelling the load makes the excess disappear without having to touch the work content. When the overburden persists once the load is levelled, it is structural and requires another treatment — rebalancing of the stations or revision of the content.