Indicators quantify and simplify phenomena to help us understand complex situations. Although indicators may be aggregates of raw and processed data, they can also be further aggregated themselves to form complex indices.

High-level decision-makers dealing with sustainable development issues—government ministers, foundation executives, heads of corporations—routinely call for a manageable number of indices that are easy to understand and use in decision-making. Many voice their desire for a single index to compete with the broad appeal and enormous political power of the gross domestic product, a commonly used index of economic growth. But these decision-makers are often impatient with the argument that no single number should be relied upon to assess something as complex as sustainable development.

Most indicator experts believe that searching for a single index is something akin to the quest for the unicorn. It is a myth to think that a single number—even one that vastly improved upon the GDP as a proxy for overall national well-being—could have any real functional value as a policy tool. But the attempt to create an SDI, a sustainable development index at the national level, may prove useful even if it fails because it might force a disciplined effort at presenting the complexity of sustainable development in a simplified form. Even a modestly successful effort to produce a small set of indices could have the effect of introducing policy and decision-makers to the goal of sustainable development.

A sustainable development index will take advantage of several important principles:

• That different types of measures can, in fact, be aggregated into a comprehensive index of sustainable development.

• That higher-level aggregation should signal the relative sustainability of a state or trend, rather than simply displaying the numerical data in a different form.

• That a vast array of complex information can be reduced to a simple presentation—but in a way that compels the user to further explore the complex details of sustainability, and draws links from individual actions to the whole of society.

• That a framework for sustainability indicators must be able to grow and adapt to society's ever-increasing understanding and sophistication in each element of the framework.