We are surrounded by quantitative indicators. They are one of the main channels whereby the citizen interacts on a day-to-day basis with science. How can the citizen make decisions in this jungle of indicators? Indicators are not simple facts about the world, but are the products of design constructed from data, assumptions and conventions. For the citizen, the question is not so much about the truth of some indicator, but about its quality. The assessment of the quality of an indicator, particularly one that is compounded from data relating to diverse fields of activity, is a demanding task, from which value commitments cannot be kept separate. The NUSAP notational system, representing the range of attributes of scientific information, from the most quantitative to the most qualitative, can be very useful in aiding the assessment of indicators. The Ecological Footprint, one of the most popular and influential sustainability indicators, performs a diversity of functions; the discussion of its merits and limitations is correspondingly complex. The choice of a particular day (not a week, nor an hour) to mark ‘earth overshoot’ provides an illustration of the problems of precision of representation of indicators highlighted by NUSAP.