Control Chart – Full Worked Example
Example based on QC sample run daily for 10 days.
Step 1 – QC Data (%)
1.24, 1.26, 1.25, 1.27, 1.23, 1.28, 1.26, 1.25, 1.29, 1.24
Mean (x̄) = 1.257 %
Standard Deviation (σ) = 0.018 %
Step 2 – Control Limits
- UCL = x̄ + 3σ = 1.257 + (3 × 0.018) = 1.311 %
- LCL = x̄ − 3σ = 1.257 − (3 × 0.018) = 1.203 %
Any point outside these limits indicates loss of control.
Interpretation
- All points within limits → Stable process
- One point above 1.311 → Out-of-control
- 7 consecutive increasing values → Trend