An indicator is a quantitative measure which represents a complex system or phenomenon. Indicators are key elements in policies for goal formulation and evaluation of achievements, as well as simplification of information and communication. There is a long tradition of applying indicators in a broad range of different sectors and disciplines working with complex systems from financial markets to health and sustainability science. Indicators are of dual nature: they are both measures (i.e. measurement protocols), and means of communication. This dual nature creates relevant issues in the development and application of indicators.
