Indicators constitute an essential resource for policy-makers, business leaders and the general public. They assist us in making evidence-based decisions, allow comparisons to be made over time and between policies and programmes, countries and regions, social groups and industries, and contribute to increased transparency and accountability. Indicators also provide a very powerful way of communicating information. Indicators are used in many areas of social and environmental science, economics, business, and project management. We do not, however, have a common language for referring to indicators. The variety of adjectives which often accompany the word 'indicator' provides a good illustration of this: headline, principal, key, core, main, primary, synthetic, composite, contextual, operational, complementary, explanatory, input, output, outcome and process - to name just a few. Navigating a route through this jungle of definitions is made much easier when we have some signposts marking the way. While recognising the importance of preserving the heterogeneity of uses and terminology specific to each domain, this paper aims to provide a point of reference for general questions about indicators and indicator sets: why do we use them? what can they measure? and how do they represent reality? These questions will allow us to distinguish between some of the main types of indicators. This paper addresses these questions by referring to the existing literature on indicators and to concrete examples drawn from the experience of Eurostat and the European Commission as a whole, as well as other national and international institutions. This paper is the first in a series of papers on statistical indicators being published by Eurostat. While in this paper we attempt to provide a general introduction to the characteristics and typologies of indicators, papers later in the series will examine in more depth issues such as communicating with indicators and the relationship between indicators and policy-making.
