1. What is Sigma Level Calculation?

Sigma level calculation converts defect rates (measured in parts per million) into sigma values, which represent process capability in Six Sigma methodology. It indicates how many standard deviations fit between the process mean and the nearest specification limit.

2. How Does the Calculator Work?

The calculator uses the sigma level formula:

Where:

• \( PPM \) — Defect rate in parts per million
• \( \text{normsinv} \) — Inverse standard normal distribution function
• \( 1.5 \) — Standard shift factor accounting for long-term process variation

Explanation: The formula converts defect rates to sigma values by finding the corresponding z-score in the standard normal distribution and adding the 1.5 sigma shift that accounts for long-term process drift.

3. Importance of Sigma Level

Details: Sigma level is a key metric in Six Sigma quality management. Higher sigma levels indicate better process performance and fewer defects. A 6 sigma process has only 3.4 defects per million opportunities.

4. Using the Calculator

Tips: Enter the defect rate in parts per million (PPM). The value must be non-negative. The calculator will compute the corresponding sigma level.

5. Frequently Asked Questions (FAQ)

Q1: Why add 1.5 to the sigma level?
A: The 1.5 sigma shift accounts for long-term process variation and drift that occurs over time, making the sigma level more representative of real-world process performance.

Q2: What is a good sigma level?
A: Generally, 6 sigma is considered excellent (3.4 defects per million), 5 sigma is very good (233 defects per million), and 4 sigma is acceptable (6,210 defects per million).

Q3: How is PPM calculated?
A: PPM = (Number of defects / Number of opportunities) × 1,000,000. It represents defects per million opportunities.

Q4: What's the difference between short-term and long-term sigma?
A: Short-term sigma doesn't include the 1.5 shift and represents ideal conditions. Long-term sigma includes the shift and represents actual performance over time.

Q5: Can sigma level be greater than 6?
A: Yes, sigma levels can theoretically go higher than 6, representing even better process performance with fewer defects per million opportunities.