Lean vs Six Sigma vs Theory of Constraints: Differences, When to Use Each & Combined Approach

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.

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.