Statistical Process Control

Overview

SPC uses control charts to monitor process stability over time. Upper and Lower Control Limits (UCL/LCL) are set at ±3σ from the process mean. Points within limits = common cause variation (stable). Points outside or showing patterns = special cause variation (investigate). Primary charts: X-bar/R, X-bar/S, I-MR, p-chart, c-chart.

When to Use

Trigger conditions:

• Monitoring production process for stability and detecting shifts
• Setting statistically-based control limits for quality metrics
• Distinguishing normal variation from assignable causes

When NOT to use:

• For process capability assessment (use Cpk)
• For root cause analysis of known problems (use fishbone/5-why)

Algorithm