four points in zone b

**Four points in Zone B** is the **SPC warning pattern where sustained one-sided points between one and two sigma suggest centerline shift** - it is an early indicator of non-random process movement. **What Is Four points in Zone B?** - **Definition**: A pattern rule triggered when four out of five consecutive points fall in Zone B or beyond on the same side. - **Statistical Rationale**: Such one-sided concentration has low probability under pure common-cause variation. - **Detection Role**: Identifies moderate shifts that may not trigger three-sigma outlier rules. - **Rule Family**: Commonly used in Western Electric style chart interpretation. **Why Four points in Zone B Matters** - **Early Shift Warning**: Provides lead time before process mean drifts into out-of-spec territory. - **Reduced Escursion Risk**: Prompt intervention can prevent yield loss from prolonged off-center operation. - **Control Discipline**: Encourages action based on evidence rather than waiting for hard failures. - **Maintenance Signal**: Repeated patterns may indicate gradual tool degradation. - **Operational Stability**: Detecting moderate shifts preserves predictable process behavior. **How It Is Used in Practice** - **Alert Configuration**: Enable rule in SPC software with clear same-side criteria. - **Immediate Checks**: Verify recent changes in tool setup, materials, and metrology calibration. - **Follow-Up Decision**: Recenter process or launch deeper RCA depending on recurrence and magnitude. Four points in Zone B is **a high-utility intermediate SPC trigger** - it catches meaningful centerline movement before severe out-of-control conditions develop.