We make a chemical measurement mostly to help make a rational decision about a ‘target’, a particular mass of material that is of interest in manufacturing, commerce, human health, or for cultural purposes. A target might comprise for example a shipment of a raw material, a batch of a manufactured product, the topsoil in a brown-field site, or a patient's blood. Chemical analysis, like all measurement, gives rise to an inevitable degree of uncertainty in the result. But you can seldom analyse a whole target—you have to work on a sample—and sampling introduces its own uncertainty. All of this uncertainty should be taken into account in decision making.