Kanban guide · Lean foundations
Theory of Constraints (TOC): The Five Focusing Steps
The Theory of Constraints (TOC) says that every system has at least one constraint, a bottleneck that limits its output. Improving anything other than the constraint makes little difference to the whole system.
Key takeaways
- Every system has a constraint that limits its output.
- Identify, exploit, subordinate, elevate, repeat.
- WIP limits and swarming apply TOC in Kanban.
Origins
TOC was developed by Israeli physicist and management thinker Eliyahu M. Goldratt and introduced in his 1984 business novel The Goal. It has since been applied to manufacturing, projects (as Critical Chain Project Management), healthcare and software.
The five focusing steps
- Identify the constraint: the stage where work piles up.
- Exploit it: make sure the constraint never wastes time, for example by never letting it sit idle or work on defective input.
- Subordinate everything else to it: other stages pace themselves to the constraint.
- Elevate it: add capacity if the first steps are not enough.
- Repeat: once the constraint moves, find the new one, and don't let inertia become the constraint.
Drum-buffer-rope
- Drum: the constraint sets the pace for the whole system.
- Buffer: a small queue before the constraint protects it from starving.
- Rope: new work is released only at the pace the constraint can handle, similar to a WIP limit.
TOC and Kanban
- A widening band on a cumulative flow diagram usually shows the constraint.
- WIP limits act as the rope, preventing overload.
- Swarming on the constraint stage exploits it.
- Improving non-constraint stages first may just move the queue around.
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Frequently asked questions
What is a constraint in the Theory of Constraints?
Anything that limits a system's output relative to its goal, most often a stage or resource with less capacity than the others.
What is Critical Chain Project Management in the Theory of Constraints?
Goldratt's application of TOC to projects: schedules are built around resource constraints, and safety time is pooled into shared buffers rather than padded into each task.
What is the Theory of Constraints?
The Theory of Constraints (TOC) is a management approach holding that every system has at least one constraint limiting its output, so improvement should focus on that constraint.
Who developed the Theory of Constraints?
Eliyahu M. Goldratt, who introduced it in his 1984 business novel The Goal.
What are the five focusing steps of the Theory of Constraints?
Identify the constraint, exploit it, subordinate everything else to it, elevate it, and repeat when the constraint moves.
What is drum-buffer-rope in the Theory of Constraints?
A TOC scheduling method: the constraint sets the pace (drum), a buffer protects it from upstream disruption, and the rope ties the release of new work to the constraint's pace.
How do you use TOC to find a bottleneck on a Kanban board?
Look for the column where work piles up, the stage just before it that is often blocked waiting, and a widening band on the cumulative flow diagram.
What does "exploit the constraint" mean in TOC?
It means getting the most from the constraint without spending money, for example never letting it sit idle, removing non-essential work from it and checking inputs are defect-free.
What does "subordinate" mean in TOC?
It means aligning every other step to the constraint's pace, even if that leaves non-constraint resources partly idle, so work does not pile up in front of it.
How does TOC relate to Kanban?
Kanban's WIP limits act like the rope, stopping work being released faster than the constraint can handle, and its visual board makes the constraint easy to see.
In the Theory of Constraints, what is the difference between a constraint and a bottleneck?
The terms are often used interchangeably. Strictly, a bottleneck is a resource whose capacity is below demand, while a constraint can also be a policy, market limit or other factor.