Creating an index that measures the level of “equity” or “opportunity” in a community can provide a powerful framework for communicating information. An index creates a single score using multiple data sources that employ different units of measurement. The units of measurement are usually standardized – using some type of scale – then the values are added together or a ratio is calculated to create the score.
For example, an equity index could be created by rolling up a small number of key indicators -- such as access to transit, food, quality education, clean air, and housing -- into a summary score. The units of measurement for each indicator would be standardized so that the indicators could be combined into a single map. The map would show a summary score for each neighborhood in the region indicating its level of equity.
The power of this approach is illustrated by the Kirwan Institute’s Opportunity Maps. Kirwan creates an “opportunity index” made up of a core set of indicators that have been demonstrated to impact an individual or family’s chance to succeed in life. The indicator data are combined to create a composite score (called a z score) for each census tract. This composite score is used to identify the opportunity-rich and opportunity-poor census tracts in the region and to assess who has access to these communities. Kirwan’s opportunity maps have been used to shape fair housing policy and to expand access to housing choices for low-income residents.
By using one measure to represent a complex concept, indices and summary scores facilitate messaging and communications about the project’s findings. They also make it easier to compare geographic areas to one another by representing each geographic area with a single measurement.
Indices and summary scores also have potential drawbacks that are important to keep in mind. First, they can lack transparency, making it difficult for people to understand how they are calculated and what they are measuring. Second, trying to get a diverse group of stakeholders to agree on which indicators to include in an index can be challenging, time consuming, and potentially divisive. Third, mapping projects that result in a single index or summary score may not have as much flexibility and broad applicability as those that allow users to create their own customized maps drawing from a wide range of different indicators.