Control Chart & SPC
Plot process data in time order, calculate control limits and distinguish common-cause variation from signals that require investigation.
Purpose and method
Assess process stability over time with control limits and statistical process-control signals.
When to use it
Use SPC for repeated measurements where common-cause variation must be separated from special causes.
Core principle
Control limits are estimated from process behaviour, whereas specification limits come from requirements. A process can be stable but incapable, or unstable while still producing conforming parts.
What the interactive module provides
- Control chart
- UCL, center line and LCL
- Stability warnings and capability indicators
- control chart, centre line, control limits and rule-based signals
Professional verification: The module supports learning, analysis and documentation. Verify results against actual process conditions, internal standards, safety requirements and competent professional judgement before implementation.
01
Recommended workflow
Plot process data in time order, calculate control limits and distinguish common-cause variation from signals that require investigation.
- 1
Confirm the measurement and rational subgroup plan
- 2
Plot data in production order
- 3
Calculate limits from a representative stable period
- 4
Investigate special-cause signals without tampering
- 5
Recalculate limits only after a sustained process change
02
Practical example
All dimensions are within tolerance, but eight consecutive points rise. The chart identifies tool wear early enough to schedule correction before defects occur.
Calculation or verification principle
For an individuals chart, centre line is the process mean and limits are commonly estimated from moving range. Select the chart type according to data and subgroup structure.
Required inputs → Expected outputs
- Required inputs
- time-ordered data, sampling plan, subgroup definition and optional specification limits
- Expected outputs
- control chart, centre line, control limits and rule-based signals
03
Common mistakes to avoid
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Using specification limits as control limits
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Recalculating limits after every unusual point
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Reacting to every common-cause fluctuation
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Mixing different products or conditions without stratification
04
Frequently asked questions
What data do I need to use this tool?
Prepare time-ordered data, sampling plan, subgroup definition and optional specification limits.
What result does the module produce?
The module produces control chart, centre line, control limits and rule-based signals.
Can I implement the result without further review?
No. Treat the output as a structured engineering and improvement aid. Verify source data, assumptions, safety, quality, legal requirements and local process conditions with competent responsible people before implementation.
LEAN Toolbox
Open interactive tool →Control Chart & SPC
Plot process data in time order, calculate control limits and distinguish common-cause variation from signals that require investigation.