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Theory of Constraints: why the bottleneck moves
At any moment exactly one station limits what a production line can ship. Every hour of improvement spent anywhere else buys nothing. That is the whole of the Theory of Constraints, and the reason it is hard to believe until you have watched it happen.
The five focusing steps
Goldratt’s method is deliberately short enough to remember. Identify the constraint. Exploit it — stop wasting the capacity you already have there. Subordinate everything else to it, which usually means upstream stations must slow down rather than speed up. Elevate it by adding real capacity. Then go back to the first step, because you have just moved the constraint somewhere new.
The last step is the one people skip, and it is the one that matters. A constraint is not a station you find once. It is a position that keeps moving, and a line that was improved last quarter is being limited somewhere else this quarter.
Why upstream capacity is usually wasted money
The counter-intuitive half is subordination. Running a non-constraint station at full tilt does not add output; it adds inventory in front of the constraint, which costs money to hold and hides the problem. A line where every station is busy is not a well-run line, it is a line accumulating work-in-process.
This is measurable rather than philosophical. In a Flowline puzzle built around a multi-input assembly, three separate upstream over-builds — three more mills, four more camps, two more assembly stations — all measure at exactly the starting throughput, to the decimal. Feeding the starved arm instead triples it.
How to spot the constraint from instruments alone
Utilization does the work. Scale the line’s analytic throughput onto each station’s share of the bill of materials and the constraint reads exactly 100%, by construction. Every other station’s reading is the headroom you have there — which is the question a spotlight never answers, and the reason Flowline’s Challenge Mode hides its own diagnosis and leaves you the numbers.
Try it on a real line
Each of these is a fixed-seed puzzle factory in Flowline’s Challenge Mode. Every one is verified before it ships: the intended fix must win, doing nothing must fail, and a plausible wrong fix must also fail — so the puzzle isolates the idea it claims to teach. They run in your browser.
- The Obvious Suspect — Ship furniture at ≥ 20/min Chase one constraint through three stations. The obvious suspect really is guilty — and then it moves.
- The Long Pole — Ship premium furniture at ≥ 15/min A multi-input assembly gated by its weakest feeder. Three plausible upstream over-builds measure zero improvement.
- Two Suspects — Ship furniture at ≥ 38/min Two stations tied at exactly the same binding ratio. Buying either alone never moves the reading, however many you buy.