Lean Six Sigma: the speed of Lean and the precision of Six Sigma

In one line

Lean Six Sigma uses Six Sigma's problem-solving framework (DMAIC) together with Lean's flow and waste tools, to improve speed and quality at the same time.

What it is

The paper from the Universidad Autónoma de Ciudad Juárez separates the three ideas:

• Lean aims to reduce and eliminate the eight wastes and to work according to demand, with takt time and the pull system, so that processes can adapt to change. It delivers quick results, above all in productivity, with tools such as the value stream map, takt time, standardized work and lead time analysis.
• Six Sigma focuses improvement on reducing process variation; the target is at most 3.4 defects per million opportunities, and its results are long-term, visible in annualized savings.
• Lean Six Sigma uses the DMAIC cycle (define, measure, analyze, improve, control) with Lean tools such as cellular manufacturing, JIT, kanban and heijunka, to reduce waste and, as a result, improve quality.

The authors also propose a variant they call Lean-Sigma, aimed at solving problems in the shortest possible time (results in at least four weeks) and not only at achieving annualized savings. Its five steps: identify and measure the problem; analyze and find the root cause; develop the solution; verify the solution; and establish a control plan.

Real example

This is the paper's own case, on a gear and chain assembly line at a company in Ciudad Juárez (Chihuahua), from October 4 to December 17, 2021.

The problem. The customer had gone from asking for weekly shipments to three per day, and the line could not keep up: the target was 90% on-time deliveries, and the indicator fell from 94% in August to 66% in December.

Identify and measure. With an initial sample: 130 defective parts out of 1,130 (DPPM of 115,044, initial sigma level of 2.6), productivity of 1.8 parts per minute per person, average lead time of 26.07 seconds and only 2 of 3 shipments met.

Analyze and root cause. A brainstorming session with production, manufacturing engineering and quality, a filtering step using the nominal group technique, and 5 whys left three candidate causes: line balancing, lack of poka-yokes and fixtures. A statistical test with 25 measurements per cause showed that rebalancing was the one that moved lead time the most (from 26.07 to 20.73 seconds, a difference of 5.34), compared with poka-yokes (from 26.07 to 23.87) and fixtures (no significant difference).

Develop the solution. Takt time was 6 seconds and two of the four stations exceeded it with more than 7 seconds, while the other two were below 5: the first ones assembled 5 components and the last ones only 2. The station calculation showed that six operators and six stations were needed, but adding people was not feasible, so the distribution of components among the existing stations was rebalanced. See line balancing and takt time.

Verify and control. A confirmation run was carried out and the new distribution was standardized. Over five weeks of follow-up, shipment compliance was 100%.

Result. 33 defective parts out of 1,250 (DPPM of 26,400, sigma level of 3.4); productivity from 1.8 to 2.5 parts per minute per person; lead time from 26.07 to 17 seconds; and the problem solved in 24 days (3.5 weeks).

How to put it together

Step	Guiding questions	Typical tools
1. Identify and measure	What is the problem, with which indicator, and what value does it have today?	Sigma level, productivity, lead time, compliance
2. Analyze and root cause	What possible causes are there and which one moves the indicator the most?	Brainstorming, 5 whys, statistical test
3. Develop the solution	What change attacks that cause without over-investing?	Line balancing, takt time, JIT, poka-yoke
4. Verify	Did the indicator improve in a sustained way?	Confirmation run
5. Control plan	How do you avoid going back?	Standardization, follow-up

Benefits

• It combines speed (Lean) and statistical rigor (Six Sigma) in a single method.
• It lets you compare causes with data before spending on a solution.
• It leaves behind a control plan and not just a one-off improvement.

Limitations to keep in mind

• A single case, like the one described, does not prove that the methodology works in any plant: it is a case study of one particular line.
• The improvements it reports (for example, the increase in productivity) are calculated by the authors with their sampling data; it is worth measuring in your own plant.
• It requires knowing both sets of tools: someone who only masters Lean may skip the statistical validation, and someone who only masters Six Sigma may overlook the flow.

In summary

Lean Six Sigma is not a third, separate methodology: it means using Lean to see the flow and the waste, and Six Sigma to measure and validate, organized by DMAIC.