In any Six Sigma project, metrics are essential. They help you understand process performance, uncover inefficiencies, and drive continuous improvement. Without clear metrics, it’s hard to identify problems or measure success.
This guide covers the most important Six Sigma metrics:
- DPMO (Defects Per Million Opportunities)
- DPU (Defects Per Unit)
- FTY (First Time Yield)
- RTY (Rolled Throughput Yield)
- COPQ (Cost of Poor Quality)
- Sigma Level
- Cpk and Ppk (Process Capability Indices)
By the end of this article, you’ll know how to use each metric and how to calculate them with real-world examples.
Why Six Sigma Metrics Matter
Six Sigma is all about improving quality and reducing variation. To achieve that, you need data. Metrics give you that data. They quantify defects, measure yield, and track costs related to poor quality.
When teams use metrics consistently, they are able to:
- Make better decisions
- Prioritize critical improvement projects
- Monitor performance over time
- Justify investments in quality initiatives
Let’s explore the core Six Sigma metrics one by one.
DPMO – Defects Per Million Opportunities
DPMO measures how many defects occur in every one million opportunities for error. It’s one of the most widely used Six Sigma metrics because it accounts for complexity. More components or steps mean more chances for something to go wrong.
Formula:
Example:
Imagine a factory produces 1,500 laptops daily. Each laptop has 7 key functions that must be defect-free (screen, battery, keyboard, ports, Wi-Fi, speakers, and charging).
- Total laptops = 1,500
- Opportunities per unit = 7
- Total defects found = 105
Interpretation:
A DPMO of 10,000 means the process produces 10,000 defects per million opportunities.
Sigma Level Conversion:
Use this table to estimate the Sigma Level from the DPMO:
| Sigma Level | DPMO | Yield (%) |
|---|---|---|
| 6 | 3.4 | 99.99966% |
| 5 | 233 | 99.98% |
| 4 | 6,210 | 99.38% |
| 3 | 66,807 | 93.32% |
| 2 | 308,537 | 69.15% |
| 1 | 691,462 | 30.85% |
As DPMO decreases, your process moves closer to Six Sigma excellence.
DPU – Defects Per Unit
DPU tracks the average number of defects found in each unit. Unlike DPMO, it doesn’t consider how many potential defects each unit could have. It treats each product equally.
Formula:
Example:
You inspect 1,000 smartphones and find 120 defects:
This means that, on average, there are 0.12 defects per unit.
When to Use DPU:
DPU works best when:
- The product has only a few features
- You’re comparing across different time periods
- You want a simple view of defect trends
Keep in mind, DPU doesn’t show the full picture in complex products with many opportunities for failure.
FTY – First Time Yield
First Time Yield (FTY) measures the efficiency of a process step. It shows the percentage of units that pass without needing rework or repair. It helps pinpoint which step in the process causes the most trouble.
Formula:
Example:
At a packaging station:
- 1,000 boxes enter
- 950 leave without defects
- 50 require rework
Interpretation:
FTY tells you that 95% of units pass this step the first time.
Sample FTY Table:
| Process Step | Units In | Good Units | FTY |
|---|---|---|---|
| Cutting | 1,000 | 950 | 95.0% |
| Welding | 950 | 900 | 94.7% |
| Painting | 900 | 850 | 94.4% |
Track FTY at each step to identify weak points in your process.
RTY – Rolled Throughput Yield
Rolled Throughput Yield (RTY) looks at the entire process. It tells you how likely a product is to pass through all steps without a single defect. RTY multiplies the FTY of each process step.
Formula:
Using Previous FTY Table:
Interpretation:
Only 84.9% of units pass through all three steps without any rework.
RTY drops quickly as more steps are added. Even high FTYs can result in a much lower RTY.
COPQ – Cost of Poor Quality
Cost of Poor Quality (COPQ) shows how much money your company loses due to defective products and processes. COPQ includes four key areas:
- Internal Failures – Rework, scrap, downtime
- External Failures – Returns, complaints, warranty claims
- Appraisal Costs – Inspections, audits, testing
- Prevention Costs – Training, process improvement, quality planning
Formula:
Example Breakdown:
| Category | Cost |
|---|---|
| Internal Failures | $30,000 |
| External Failures | $15,000 |
| Appraisal Costs | $10,000 |
| Prevention Costs | $5,000 |
| Total COPQ | $60,000 |
Interpretation:
You’re losing $60,000 due to poor quality. This cost can make up 20–30% of total sales in some companies.
Reducing COPQ improves both profit margins and customer satisfaction.
Sigma Level – Your Quality Scorecard
The Sigma Level measures how close your process is to perfection. It shows how many standard deviations fit between the mean and the nearest spec limit.
Higher Sigma Levels mean fewer defects.
Quick Reference Table:
| Sigma Level | DPMO | Yield % |
|---|---|---|
| 6 | 3.4 | 99.99966% |
| 5 | 233 | 99.98% |
| 4 | 6,210 | 99.38% |
| 3 | 66,807 | 93.32% |
| 2 | 308,537 | 69.15% |
| 1 | 691,462 | 30.85% |
Estimating Sigma Level:
You can estimate Sigma Level from yield using statistical Z-tables or software like Minitab. Most Six Sigma teams aim for 4.5 Sigma or better to remain competitive.
Cpk and Ppk – Process Capability Indices
Cpk and Ppk compare your process output to specification limits.
- Cpk: How well a process stays within specs when it’s in control
- Ppk: How well it performs overall, even with variation or drift
Both use standard deviation and mean, but Cpk assumes process stability, while Ppk does not.
Formulas:
Where:
- USL = Upper Spec Limit
- LSL = Lower Spec Limit
- μ or x̄ = Process Mean
- σ or s = Standard Deviation
Example:
A part must be between 19.8 and 20.2 mm.
- Mean = 20.0 mm
- Std Dev = 0.1 mm
Interpretation:
This process is not capable. Cpk must be ≥ 1.33 to meet Six Sigma standards.
Capability Index Table:
| Cpk/Ppk Value | Interpretation |
|---|---|
| ≥ 2.0 | World-class |
| 1.66 – 2.0 | Excellent |
| 1.33 – 1.66 | Good – Acceptable in most cases |
| 1.0 – 1.32 | Marginal – Needs improvement |
| < 1.0 | Not capable |
Summary Table of Key Six Sigma Metrics
| Metric | Description | Formula Summary | Ideal Range |
|---|---|---|---|
| DPMO | Defects per opportunity | (Defects / (Units × Opportunities)) × 1M | Lower is better |
| DPU | Defects per unit | Defects / Units | Lower is better |
| FTY | First-pass yield per step | Good Units / Total Units | Close to 100% |
| RTY | Total yield across all steps | Product of FTYs | Close to 100% |
| COPQ | Total cost of poor quality | Sum of 4 categories | Minimize |
| Sigma Level | Distance from perfection | Derived from DPMO or yield | 6 is ideal |
| Cpk/Ppk | Process capability vs specs | See formulas above | ≥ 1.33 preferred |
Conclusion
Six Sigma metrics do more than just track performance. They guide decisions, drive accountability, and foster continuous improvement. When used correctly, they align teams around a common goal—delivering defect-free products and services.
Whether you’re starting your first DMAIC project or managing global operations, these metrics offer the insights you need to improve quality, reduce costs, and enhance customer satisfaction.
