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.