"For EPA's Report on the Environment, an indicator is a numeric value derived from actual measurements of apressure, state or ambient condition over a specified geographic domain, whose trends over time represent or draw attention to underlying trends in the condition of the environment. Key indicator criteria are:

• the indicator is useful
• the indicator is objective
• the indicator is transparent and reproducible
• the underlying data is characterized by sound collection methodologies, data management systems to protect its integrity, and quality assurance procedures
• data are available to describe changes or trends
• the data are comparable across time and space, and representative of the target population.