Sigma Shift (1.5)

Introduction: Sigma Shift (1.5)

The sigma shift is a convention in Six Sigma that says long term performance is usually worse than short term performance. The rule of thumb is to assume a fixed gap between the two. This helps teams translate a recent capability result into what customers may see over time.

Background

The idea comes from early Motorola studies. Engineers saw that processes tend to drift as tools wear, materials vary, and conditions change. Short studies taken under stable conditions looked better than field results collected across months. To compare plants and products on one yardstick, practitioners adopted a standard allowance for drift. This became known as the one point five sigma shift.

Key Elements or Features

• Short term versus long term. Short term reflects within shift variation with controlled inputs. Long term includes shifts, seasons, and other sources of change.
• Fixed adjustment. The convention applies a single allowance to move from the short term view to the long term view.
• Communication aid. The shift creates a simple way to link capability levels to expected defect rates that customers may experience.
• Benchmarking. Organisations can compare suppliers or plants even when their data windows differ.
• Controversy. A fixed allowance is not universal. Real drift depends on the process, the measurement system, and the controls in place.

Applications or Examples

• Target setting. Convert a recent capability snapshot into a long term expectation for service or warranty planning.
• Design reviews. Stress test margins by asking how performance holds up if the process drifts.
• Supplier management. Put short term certificates into context when judging year round risk.
• Improvement planning. Decide where tighter controls or maintenance will reduce drift.

Relevance / Impact

The sigma shift is useful for quick planning and clear messages. It prevents overconfidence from very clean pilot data. Use it with care. When practical, estimate your own drift from control charts, history, and gauge studies, and report both short term and long term views.

See also

• Long-Term Sigma
• Short-Term Sigma
• Six Sigma
• Sigma Level