Control Charts & Statistical Process Control (SPC)
Statistical Process Control (SPC) ensures laboratory measurement systems remain stable, repeatable, and compliant with ISO 17025 requirements.
What is a Control Chart?
- Graphical monitoring tool
- Tracks measurement over time
- Detects drift and instability
- Separates random vs systematic error
Main Types of Control Charts
X-bar Chart
- Monitors process mean
- Used for replicate samples
- Detects drift over time
R Chart
- Monitors variability
- Shows spread of results
- Detects instability in repeatability
Control Limit Calculation
- Mean (x̄)
- Standard deviation (σ)
- Upper Control Limit (UCL = x̄ + 3σ)
- Lower Control Limit (LCL = x̄ − 3σ)
3-sigma limits capture 99.7% of normal distribution.
Westgard Rules (Quality Control Rules)
- 1-2s warning rule
- 1-3s rejection rule
- 2-2s rule
- R-4s rule
- 4-1s rule
- 10x rule (systematic bias)
Violation of rules indicates need for corrective action.
Common SPC Signals
- Trend (7 points increasing/decreasing)
- Sudden shift
- Point outside control limits
- Increased variability