Quality control (QC) is the ongoing process of monitoring a measurement system to make sure it stays accurate and consistent over time. Levey-Jennings charts are often used for QC to spot drift, trends or out-of-control points, often using Westgard rules to decide when a run should be flagged or rejected.
The Westgard Rules are:
- 1S3
- One value beyond 3σ from the mean.
- 2S2
- Two consecutive values either greater than, or less than, 2σ from the mean.
- RS4
- A difference between consecutive values greater than 4σ.
- 4S1
- Four consecutive values greater than, or less than, 1σ from the mean.
- 10X
- Ten consecutive values all greater than, or less than, the mean.
Below is an example of a Levey-Jennings chart with Westgard rule violations highlighted. The data was processed in Azure Databricks and displayed in Power BI. The AI interpretation was generated using a GPT-5 mini model deployed in Azure Foundry.
