In selecting environmental indicators, it is important to have clear selection criteria. Previously developed criteria are available from several sources including the Environmental Monitoring and Assessment Program (EMAP), the International Joint Commission for the Great Lakes (IJC), and the Intergovernmental Task Force on Water Quality Monitoring (ITFM). The choice of selection criteria depends in part on the intended use of the indicators. Therefore, the list of criteria suggested here, in Table A-1, has been adapted from other sources (primarily the ITFM criteria).
The selection criteria are grouped based on considerations of validity, interpretability, timeliness, understandability, and cost considerations. These considerations include the following:
Indicators should be valid measures of the valued attribute. Validity is a qualitative association between the concept embodied in the value and the measurable quantity represented by the indicator. Validity is established if there is a close scientific or logical link between the indicator and the the value. Three factors are listed in Table A-1 that contribute to a close logical link between the indicator and the valued attribute. First, indicators that have wide scope provide a balanced measure of the valued attribute. Second, indicators that respond to the cumulative effect of multiple stressors will be more generally applicable than those that are responsive to only a few stressors. Third, indicators that are highly correlated with other measures of the valued attribute will be generally applicable to the environmental system being measured. Indicators must be sensitive enough to measure changes over a reasonable time but not so sensitive that they fluctuate substantially between time periods. The signal-to-noise ratio for an indicator is in part determined by the data used to assess the indicator. Expert knowledge and peer review can be used to assess the sensitivity of different indicators.
Indicators should be interpretable in terms of the end point in the assessment process. They should be able to distinguish unacceptable from acceptable environmental conditions.
Timely indicators that anticipate future changes in the environment are preferred over those that are not anticipatory. To the extent that an indicator does not anticipate future conditions, the indicator with the least time lag would be preferred. The time lag depends on both the characteristics of the indicator and the time lag between the data collection and when the data are available to calculate the indicator.
Indicators should be understandable by the public and perceived as relevant. Understandability is in part a characteristic of the indicator and in part a function of how the indicator is presented. EPA may need to educate the public.