The effect of an influence quantity on the measurement process is an interference that generates a systematic error. In clinical chemistry, depending on the molecular entity responsible for an interference, influence quantities may be classified as endogenous or exogenous. (Clinical Chemistry)

This means that there has to be a specified relationship between the influence quantities and the measurand. This relationship is known as the mathematical model. This is an equation that describes how each influence quantity affects the value assigned to the measurand. (labnews.co.uk)

By using the measurement model built to approximately compute the sensitivity coefficient that reflects the uncertainty propagating law of each influence quantity, it is possible to evaluate the result of indirect measurement and its uncertainties. (CAT.INIST)