A proxy is an indirect measure of the desired outcome which is itself strongly correlated to that outcome. It is commonly used when direct measures of the outcome are unobservable and/or unavailable. An organization should use a proxy measure when there is little or no data available about the program being implemented, but the outcome the program is designed to influence has an existing and commonly accepted proxy.
Proxy measures can be powerful tools for governments who do not have the exact data they want, but know the outcome they are trying to achieve. Infant mortality rates, for example, are a direct measure of healthcare quality but are also a proxy for the economic and social welfare of a community. The unemployment rate is a direct measure of unemployment, but is also a proxy for the overall state of our economy.