Linearity means how much the calibration curve deviates from the straight reference trend. Linearity calculates the absolute maximum error in the whole measuring range; in this case, it is called “integral linearity error”. In the case of instruments used for comparison, so that they can be reset to a point on the instrument measuring range, it is more useful to evaluate the “differential linearity” as the ratio between the variation of the instrument’s indication and the variation of the reference sample used for comparison (see graph below).