Overall equipment effectiveness: the three honest questions

A line manager proudly reports that the machine ran all shift. True enough. But it spent an hour crawling at half speed, and a tenth of what it made had to be scrapped. Running is not the same as performing well, and to judge a machine honestly you have to ask more than one question.

Overall equipment effectiveness, or OEE, asks three. It is a single percentage for how well a piece of equipment turns the time you give it into good product, and it works on one machine, a cell, a line, or a whole plant.

The three honest questions

OEE multiplies three factors, and each answers a plain question:

• Availability asks: when we wanted it to run, was it running? It sets the hours the asset was actually operating against the hours it was scheduled to operate.
• Performance asks: when it was running, did it run at full speed? It sets the actual rate against the best rate the equipment can reliably hold.
• Quality asks: of what it made, how much was right first time? It sets the good, first-pass output against the total produced.

OEE = Availability × Performance × Quality

Why three good numbers make a humbling one

The multiplication is the whole point, and it is brutal in a way that surprises people. Because the three factors multiply rather than average, three healthy-looking numbers collapse into a sobering one. Run at full availability but let performance and quality each slip to seventy-five per cent, and your OEE is barely over half, even though nothing was technically "broken". A tool running at half speed, as one authority puts it, does the same damage to your output as a tool that is down half the time.

The world-class benchmark is widely quoted at around eighty-five per cent, and it is worth seeing where that comes from:

OEE = 0.90 × 0.95 × 0.99 = 0.846, about 85%

Roughly ninety per cent availability, ninety-five per cent performance and ninety-nine per cent quality, each of them excellent, multiply to a shade under eighty-five. Every factor has to be strong at once to clear the bar, which is why plants that have never measured often discover they sit somewhere in the forties or fifties: the losses they had stopped noticing were multiplying quietly all along.

The six big losses

OEE earns its keep because it does not just score you, it points at where the time went. The classic framework breaks the losses into six, two behind each factor. Behind availability sit breakdowns and the setup-and-adjustment time of changeovers. Behind performance sit idling and minor stoppages, the small jams and brief halts that never get logged, and reduced speed, running below the rated rate. Behind quality sit defects and rework, and the startup or yield losses you take while a process settles after a stop. Almost any loss a machine suffers lands in one of those six, which is what turns OEE from a verdict into a diagnosis: a low score always decomposes into a specific, addressable cause.

Why availability here is not calendar time

This is the part people most often get wrong. A machine might sit idle all weekend because there were no orders, no raw material, or it was a holiday. That is not a maintenance failure and not the asset's fault, so OEE refuses to punish the asset for it. The clock starts from scheduled time, not the full year at twenty-four hours a day. You take total available time, remove the idle time with no demand or no plan to run, and measure against what is left. One sensible exception: if planned maintenance happens during hours the plant was not scheduled to run anyway, those hours are added back. And because availability is the most subjective of the three factors, two plants must agree exactly which hours they count before they compare OEE, or they are not measuring the same thing.

OEE and its bigger cousin, TEEP

There is a related measure worth knowing, because the two are easily confused. OEE starts the clock at scheduled time, the hours you actually intended to run. Total effective equipment performance, or TEEP, starts it at every hour on the calendar, and so folds in a fourth factor, utilisation, that captures the hours you never scheduled at all. TEEP is therefore always the lower number, and it answers a different, more strategic question: not "how well did we run when we meant to?" but "how much of the total capacity we own are we actually using?" A low OEE is a maintenance and operations problem; a low TEEP sitting beside a healthy OEE is often a business decision about demand, not a failing on the floor.

Where it can mislead you

• Performance can never exceed 100%. It is deliberately capped, so a best-speed set too low cannot inflate the score.
• OEE is a lagging indicator. It tells you what already happened and where the losses are, not what it will cost to fix them.
• It is not a maintenance scorecard. Availability, speed and quality are shaped by operations, scheduling and materials as much as by maintenance, so no single department owns the number; treat it as a team sport.
• Do not compare OEE across different processes or plants. Measure a given asset against its own history; ranking one line against another usually compares definitions, not performance.
• Chasing one factor at the expense of another is a trap. Push speed or availability while quality quietly slips, and the score can look flat while the operation gets worse.

OEE asks three honest questions, was it available, was it fast, was it right, and multiplies the answers into one number you cannot easily fool. Master the first question, the one about scheduled time, learn to read the six losses behind the score, and you have one of the few measures that tells you not just how well you are doing but exactly where to look next.