A function which relates the measured value ${\hat{C}}_{\text{a}}$ to the instrument reading, $X$, with the value of all interferants, $C_i$, remaining constant. This function is expressed by the following regression of the calibration results. $${\hat{C}}_{\text{a}}=f\left(X\right)$$ The analytical function is taken as equal to the inverse of the calibration function.