January 4th, 2023

“Lean” and “Six Sigma'' are often mentioned together. In practice, they are complementary in process improvement efforts. Lean focuses on reducing waste, while Six Sigma focuses on reducing defects and variations. Generally, Six Sigma methods are more analytical compared to lean - there is a heavy emphasis on quantitative data analysis methods such as hypothesis testing, regression, and design of experiments.

Let’s take a high level view at Six Sigma and how it works together with lean.

Six Sigma overview

What’s in a name?

Six Sigma was developed by Motorola in 1986 with focus to reduce defects by reducing process variation. In statistics, the Greek letter σ (sigma) represents the standard deviation of a population. Here, the “sigma” level indicates the level of variability, and consequently, quality, of a process. Processes that are at “six sigma” quality are expected to have 3.4 defects per million opportunities (DPMO).

A Standard set of tools for Process Improvement

Six Sigma has a defined body of knowledge and certification levels. There is no central certification body, however. Certification can be obtained at the following levels:

Master Black Belt: Trains and coaches black and green belts. Oversees six sigma program
Black Belt: Lead and manage projects. Trains and coaches project teams.
Green Belt: Lead or support projects
Yellow Belt: Has basic understanding and participates in projects.

The certification levels indicate that the belt holder has the knowledge and skill set for problem solving following the Six Sigma framework. Leaders who are looking for talent with hands-on experience applying six sigma should ask for it in the job description.

Key Terms & Concepts

There are a plethora of concepts and terms in the Six Sigma Body of Knowledge. We do not attempt to cover all of them here. The reader should consider the following a primer and seek out additional materials and coursework to deepen their understanding.

For a list of how some of the six sigma methods translate to process development activities, see our guide on what to look for in a manufacturing engineer.

DMAIC method

Define, Measure, Analyze, Improve, Control. A Six Sigma project goes through these five stages.

Voice of the Customer

The Voice of the Customer (VOC) are the customer specifications (what the customer is willing to accept).

This can be specified by:

Lower Specification Limit (LSL)
Upper Specification Limit (USL)
Target Value (or Nominal Value)

Example: A small cup of hot coffee bought at diner is expected to have:

Temperature between 70 to 85C (77C is best)
Quantity: at least 120ml but no more than 140 ml
Not too weak but not too strong
Comes in a sturdy mug with room for milk and sugar

Voice of the Process

Performance of a process over a period of time.

Example: The volume of coffee poured manually while serving coffee.

How good am I at pouring the same amount every time?
How has it changed over three months?

In the simplest terms, process capability is a measure of how the voice of the process compares with the voice of the customer. In essence, “is the process capable of producing defect-free products? “

To improve process capability, Six Sigma practitioners will focus on 1) location, by centering the process, and 2) spread, by reducing the variation. The improvement process is where the synergy with lean happens - how do we make defects easy to detect? How do we do it right the first time? How do we empower the teammates closest to the process to take ownership and drive the improvement to fruition? With a team steeped in lean, where people are used to seeing and learning together and focusing on value, these questions aren’t hard to answer.

These improvements are rarely “one and done”, however. Equipment condition, environmental changes, raw material variations and other factors can all cause the process to drift over time. The team must keep a close eye on the process behavior to ensure the process remains stable in the “monitor” phase. How does one distinguish unexpected, “special causes” that require intervention, from “common causes” that are characteristic of the inherent process? A subtopic in Six Sigma called Statistical Process Control focuses on this. At the same time, lean principles guide the human aspect of process monitoring. Leaders should use problems as opportunities to engage the team to continuously improve quality, cost, and lead time (see “Lean Mantras”).

Start-up leaders are strongly encouraged to leverage Six Sigma methods to make data-driven evaluations and decisions while developing and refining their novel processes. This widely-practiced framework is not just for manufacturing settings. Six Sigma can and should be applied strategically during the development, optimization and the ongoing sustainment of routine processes. It should come as no surprise that continuous improvement never ends, especially when we battle entropy and Murphy’s law as companies grow in complexity and scale.

Lean & Six Sigma - A Powerful Combination for Process Development

Six Sigma overview