Resources used for production such as labor, capital, land and entrepreneurial ability are scarce, forcing individuals and societies to make choices. This can be modeled using a production possibilities curve which shows the maximum combinations of two goods an economy can produce with its available resources. The steeper the curve, the higher the opportunity cost of producing one good over another as more of the second good must be given up. Over time a country's production possibilities curve may shift outward if its resources or technology increase, reducing scarcity.