Firstly comes the use of the contribution margin. This can be defended on its normative characteristics. The unit contribution margin is a first approximation of the effect on the bottom line of a change in volume of activity of one unit of output. If we sell one more units we expect the operating income to rise by the amount of one unit of contribution margin (or if we sell one unit fewer it will fall by one unit of contribution margin). It is therefore a normatively correct report in that it is what we should do to best report the effect of a sales volume change. However, despite its normative superiority, there is evidence that the other two approaches are also used in practice.