Control Chart (Shewhart)
Details
- Full Name
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Shewhart Control Chart
- Also known as
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Process Control Chart, SPC Chart
Core Concepts:
- Time series diagram
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Measured value plotted over time
- Centerline (CL)
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Process mean
- Upper/Lower Control Limit (UCL/LCL)
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Typically at ±3σ from the mean
- Zones A/B/C
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Division into 6 areas (each 1σ wide) for pattern recognition
- Common Cause Variation
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Inherent, random fluctuation of a stable process
- Special Cause Variation
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Assignable, correctable deviation
- Chart Types
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X-bar Chart: Subgroup means
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R-Chart: Subgroup ranges
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I-MR Chart: Individual values and moving range
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p-Chart: Defect proportions
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c-Chart: Defect counts per unit
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- In-Control vs. Out-of-Control
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Core decision based on rules (Nelson, Western Electric)
- Normal distribution assumption
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Control limits are based on normally distributed data
- Key Proponent
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Walter A. Shewhart (1920s, Bell Labs / Western Electric)
- Key Work
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"Economic Control of Quality of Manufactured Product" (1931)
Relationship to Other Anchors:
- Nelson Rules
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8 rules for pattern recognition on Control Charts
- SPC
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Control Charts are the central tool of Statistical Process Control
- Six Sigma
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Control Charts are used in the Control phase of DMAIC
When to Use:
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Process monitoring in manufacturing and production
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Quality assurance using statistical methods
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Detection of process shifts and trends
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Foundation for rule-based anomaly detection in time series
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Conceptual basis — even when different terminology is used in IT monitoring