From the preceding graph, we see that the PPF is the boundary between attainable and unattainable combinations of food and helicopters. There are unattainable combinations (above the PPF) because of scarcity: for a given level of technology, finite resource quantities limit production. If a country fails to use all of its available resources and/or its best available technologies, production will correspond to a point below the PPF. Such points are inefficient because they admit the possibility of producing more of one good without sacrificing any of the other.

The problem of choice faced by all economies is illustrated by the fact that there is an infinite number of points along the PPF. Each of these is efficient in the sense of not admitting the possibility of getting more of one good without giving up some of the other. There is no simple answer to the question of which efficient point a country should produce at. Unless people's tastes are accounted for in some way, every point along the PPF can be considered as good as every other point.

PPFs are generally assumed to be concave to the origin (unlike that of the preceding graph). This shape implies that the opportunity cost grows larger and larger as the output of either good is increased. For example, if a country producing only helicopters decides to produce a little food, the cost in terms of forgone helicopters is likely to be very small. On the other hand, if the country is producing very few helicopters, the opportunity cost of a small increase in food output is likely to be quite large (a lot of helicopters). The reason that we expect such results is that resources are not uniformly productive. The most efficient way of producing an additional tonne of food would be to reallocate helicopter-industry resources with the lowest helicopter-making to farming ability ratio. Doing so would mean that the relative productivity in helicopter-making of the resources lost from the helicopter industry rises along with food output. Thus, the number of helicopters forgone increases as more food is produced.

By shifting the production possibilities frontier outward, technological improvement allows a country to expand its output without using more resources. Spurring this sort of  economic growth is currently a priority among developed nations.

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Continue to Chapter 1