There is more information we don't know than we do know for makiig most critical decisions involving risks. Our focus must be on understanding and effectively dealing with what we don't know. As a first step in achieving this focus, a classification of the types of uncertainties that must be addressed and the sources of these types of uncertainties is presented. The purpose is to provide a framework for discussion about addressing uncertainty, particularly in risk analyses.Both uncertainty and variability of information are addressed using four main classes: 1) Metrical 2) Structural 3) Temporal 4) Translational uncertainty and variability in measurement, uncertainty due to complexity, including models and their validation, uncertainty in future and past states uncertainty in explaining uncertain results.The factors that contribute uncertainty and error to these classes are identified, and their interrelationships indicated. Both subjective and objective aspects are addressed. ~~~~ ~ KEY WORDS Uncertainty; variability; measurement; value diversity; membership variability.

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In a joint undertaking by The MITRE Corporation and the EPA Office of Water Programs Operations, three indices were developed: (1) An index of water quality; (2) An index of the need for abatement action; and (3) An index of the need for water quality management planning. These indices were evaluated for some 1000 planning areas (sub basins, regional planning jurisdictions, metropolitan areas, etc.) which together cover essentially all of the area of the 48 contiguous states.