Long-Term Sigma

Introduction: Long-Term Sigma

Long-Term Sigma is a process performance metric in Six Sigma and quality management that evaluates how well a process performs over an extended period of time. Unlike short-term sigma, it accounts for real-world variation caused by shifts in operators, equipment, environment, and other factors. Long-Term Sigma provides a more realistic picture of true process capability.

Background

In Six Sigma, sigma levels are used to measure the number of defects per million opportunities (DPMO) and overall process quality. While short-term sigma reflects immediate capability under stable conditions, Long-Term Sigma captures sustained performance. It typically shows lower values than short-term sigma, as it includes natural sources of variation. This measure ensures that process improvements are sustainable, not just short-term fixes.

Key Elements / Features

• Extended performance: Evaluates outcomes across weeks, months, or years.
• Real-world variation: Includes environmental conditions, equipment wear, operator differences, and system shifts.
• Lower values: Usually lower than short-term sigma, as it incorporates broader variability.
• Sustainability check: Provides a realistic benchmark for long-term quality and reliability.

Applications / Examples

• Manufacturing: Tracking defect rates over several months to confirm process stability.
• Healthcare: Monitoring patient safety metrics across multiple years, accounting for staffing and seasonal variation.
• Banking: Evaluating service accuracy across multiple branches and long timeframes.

Relevance / Impact

Long-Term Sigma is crucial for ensuring that improvements are sustainable and not limited to short-term gains. It reflects the performance customers experience over time and helps organisations reduce risk, improve consistency, and build trust. Used alongside Short-Term Sigma, it provides a complete picture of process capability.

See also

• Short-Term Sigma
• Six Sigma
• Sigma Level
• Sigma Shift (1.5)