<P> The PSU represented by the point on the PPC where an efficient economy operates (which is obtained by tangency with the highest individual or social indifference curve, not shown in the graph) presents the priorities or choices of the modeled agent, such as the choice of having more butter produced and fewer guns, or vice versa . </P> <P> PPFs are normally drawn as bulging upwards or outwards from the origin ("concave" when viewed from the origin), but they can be represented as bulging downward (inwards) or linear (straight), depending on a number of assumptions . A PPF illustrates several economic concepts, such as scarcity of resources (the fundamental economic problem that all societies face), opportunity cost (or marginal rate of transformation), productive efficiency, allocative efficiency, and economies of scale . </P> <P> An outward shift of the PPC results from growth of the availability of inputs, such as physical capital or labour, or from technological progress in knowledge of how to transform inputs into outputs . Such a shift reflects, for instance, economic growth of an economy already operating at its full productivity (on the PPF), which means that more of both outputs can now be produced during the specified period of time without sacrificing the output of either good . Conversely, the PPF will shift inward if the labor force shrinks, the supply of raw materials is depleted, or a natural disaster decreases the stock of physical capital . </P> <P> However, most economic contractions reflect not that less can be produced but that the economy has started operating below the frontier, as typically, both labor and physical capital are underemployed, remaining therefore idle . </P>

Why is a ppf sometimes drawn as a straight line why is a ppf sometimes drawn as a curve