Linearity Check is a verification that the instrument response is proportional to the measured property across the working range — confirming that the calibration curve is linear (or follows the expected mathematical model) throughout the measurement range, without curvature, saturation, or other nonlinearities.

Linearity Check Method

• Standards: Measure 5-10 standards spanning the full range — including near-zero and near-maximum values.
• Residuals: Plot regression residuals vs. concentration — random scatter indicates linearity; systematic patterns indicate non-linearity.
• R²: Correlation coefficient for linear fit — R² > 0.999 typically indicates acceptable linearity.
• Mandel Test: Statistical test comparing linear vs. quadratic fit — determines if curvature is statistically significant.

Why It Matters

• Accuracy: Non-linearity causes concentration-dependent bias — measurements at the ends of the range may be inaccurate.
• Range Limits: Linearity defines the usable range — detector saturation causes non-linearity at high values.
• Method Validation: Linearity is a required method validation parameter — documented in the validation report.

Linearity Check is testing the straight line — verifying that the instrument's response is proportional to the measured quantity across the full working range.