Stability, linearity, repeatability, and reproducibility

Learning objectives

• Distinguish stability, linearity, repeatability and reproducibility.
• Choose the check appropriate to each property.
• Interpret failure patterns to guide corrective action.

Four properties, four risks

• Stability — consistency of bias over time.
• Linearity — consistency of bias across the measurement range.
• Repeatability — variation within one appraiser using the same method.
• Reproducibility — variation between appraisers.

Garment-factory example

Inspectors agree on obvious puckering but disagree near the acceptance boundary — a reproducibility issue that may also be a linearity issue if boundary items sit at one end of the severity scale.

Method

1. Use a set of standards that span the measurement range.
2. Repeat measurements over time and across appraisers.
3. Plot bias by reference value and by time.
4. Fix method, fixtures, training, environment or instrument accordingly.

Common mistakes

• Blaming ‘training’ for every disagreement.
• Studying only one reference value.