Six Sigma Terms Explained: Your Glossary for Getting Started

Whether you're just getting started with Six Sigma or need a quick refresher, this glossary simplifies the core terms and tools you’ll encounter on your quality improvement journey.

1. Six Sigma

A data-driven methodology for improving processes by identifying and eliminating defects or variations. The goal is to achieve near-perfect performance — typically 3.4 defects per million opportunities (DPMO).

2. DMAIC

The core Six Sigma framework for problem-solving:

• Define: Identify the problem and goals.

• Measure: Collect data to understand current performance.

• Analyze: Identify root causes of problems.

• Improve: Implement and test solutions.

• Control: Sustain the gains with ongoing monitoring.

3. Defect

Any output or result that does not meet customer requirements or specifications.

4. Variation

The inconsistency in a process, product, or outcome. Six Sigma aims to reduce variation to improve quality and predictability.

5. Process Capability (Cp, Cpk)

A statistical measure of a process’s ability to produce outputs within specification limits:

• Cp compares spread (variation) to the specification width.

• Cpk accounts for how centered the process is within those limits.

6. Critical to Quality (CTQ)

Key measurable characteristics that are critical to the customer’s satisfaction. CTQs help translate customer needs into process requirements.

7. SIPOC

A high-level process mapping tool showing:

• Suppliers

• Inputs

• Process steps

• Outputs

• Customers used during the Define phase to frame project boundaries.

8. Voice of the Customer (VOC)

Captures what customers value, need, or expect—often through surveys, interviews, or complaints.

9. Root Cause Analysis (RCA)

A method to identify the fundamental cause of a defect or problem. Tools include 5 Whys and Fishbone (Ishikawa) diagrams.

10. Control Chart

A graphical tool used to monitor process stability over time. It helps distinguish between common cause variation (normal) and special cause variation (requires action).

11. Gage R&R

A tool used to assess the reliability and precision of a measurement system by evaluating repeatability and reproducibility.

12. Y = f(x)

A core Six Sigma concept meaning that the output (Y) of a process is a function of inputs (x). In short, “fix the x’s to improve the Y.”

13. FMEA (Failure Modes and Effects Analysis)

A structured approach to identify and prioritize potential failures in a process, and plan preventive actions.

14. Lean

Often integrated with Six Sigma, Lean focuses on eliminating waste and increasing flow, improving speed and efficiency.

15. Green Belt / Black Belt / Master Black Belt

Six Sigma roles and certification levels:

• Green Belt: Leads projects part-time.

• Black Belt: Full-time project leader and analyst.

• Master Black Belt: Coaches Belts and leads strategic deployment.

16. DPMO (Defects Per Million Opportunities)

A standard Six Sigma metric that measures how many defects occur in one million opportunities for error. It is calculated using the formula: DPMO = (Defects / (Opportunities × Units)) × 1,000,000

Used to benchmark process performance and compare across industries.

17. Histogram

A bar chart that displays the distribution of data over a range. It shows how often each value or range of values occurs, making it easier to visualize central tendency, spread, and skewness of the data — key for identifying patterns and variation in a process.

18. Box Plot

Also known as a box-and-whisker plot, it displays the median, quartiles, and potential outliers in a dataset.

• The box shows the interquartile range (IQR: Q1 to Q3).

• The line inside the box shows the median.

• The “whiskers” show variability outside the upper and lower quartiles. Box plots are valuable in Six Sigma for comparing data sets and identifying outliers or shifts in a process.

19. Poka-Yoke (Error Proofing)

A Japanese term that means “mistake-proofing.” Poka-Yoke refers to designing processes, tools, or systems in such a way that errors are prevented before they happen or easily detected and corrected if they do. Examples:

• A USB plug that only fits one way

• A washing machine that won’t start if the door is open