linearity check

**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.