Chapter 3: Variation in Processes and Rational Subgrouping
Abstract: Variation in processes is categorized into common cause (inherent, random) and special cause (assignable, external), with Rational Subgrouping being a key statistical method to separate these by grouping items produced under similar, constant conditions, minimizing within-subgroup variation to highlight larger between-subgroup shifts on control charts, making them sensitive to process changes . This technique creates "snapshots" of the process, allowing engineers to diagnose stability by comparing variation within a subgroup (common cause) to variation between subgroups (potentially special causes). Key Concepts Explained Common Cause Variation: The natural, expected variability within a stable process, representing random fluctuations. Special Cause Variation: Variation due to identifiable, external factors (e.g., machine malfunction, new operator, shift change) that make a ...