Resources used for production such as labor, capital, land and entrepreneurial ability are scarce. Because of this scarcity, societies must make choices about how to allocate these scarce resources between different goods and services. This can be modeled using a Production Possibility Curve which shows the maximum possible output combinations of two goods an economy can produce with its available resources and technology. The steeper the slope of the PPC, the higher the opportunity cost of producing one good over another. Over time, a country's PPC may shift outward as its resources and technology improve, allowing it to produce more of both goods.