Lean, Six Sigma and the Theory of Constraints (TOC) are all process-improvement approaches, but they focus on different problems. Lean emphasizes flow and waste reduction; Six Sigma emphasizes variation and defect reduction; TOC emphasizes the system constraint that limits throughput.
Comparison
| Method | Primary focus | Typical question | Common tools |
|---|---|---|---|
| Lean | Flow, waste, lead time | What prevents value from flowing smoothly? | VSM, 5S, pull, standard work, kaizen |
| Six Sigma | Variation, defects, process capability | What causes inconsistent or defective output? | DMAIC, statistical analysis, control charts, capability |
| TOC | System constraint / throughput | What limits the performance of the entire system? | Five focusing steps, buffer management, drum-buffer-rope |
TOC Five Focusing Steps
- Identify the constraint.
- Exploit the constraint using existing capacity effectively.
- Subordinate other activities to the constraint.
- Elevate the constraint if more capacity is required.
- Repeat when the constraint moves.
Supply-Chain Examples
- Lean: reduce PR-to-PO waiting by removing duplicate approvals.
- Six Sigma: analyze why supplier lead time varies widely around the average.
- TOC: improve a packing station that limits the throughput of the entire warehouse.
Why the Methods Can Work Together
A warehouse may have one packing constraint, significant travel waste and high variation in receiving. TOC helps identify where system throughput is limited; Lean can remove wasted motion and waiting; Six Sigma can reduce unstable process variation. The business problem should determine the method—not the other way around.
Related: Lean Operations, Variation Analysis, and Value Stream Mapping.
Interview Question
Question: When would you use TOC before Lean or Six Sigma?
Answer: When the immediate business problem is system throughput and one stage clearly limits the whole process. Improving non-constraints may create local efficiency without increasing end-to-end output.
Reference
American Society for Quality — comparison of Lean, Six Sigma and Theory of Constraints; ASQ Six Sigma resources.
