Many environmental assessments have compiled large amounts of uninterpreted data which are not geared to the needs of decision makers. The scientific findings of an environmental assessment depend on the spatial boundaries of the system and the time horizon of the analysis. A dynamic analysis is preferable if the scientific data and analytical resources needed to support it are sufficient; otherwise a simpler, static assessment might be more valid. Many environmental assessments methods fall into a fact/value trap by attempting to derive summary scores that combine indicators of the magnitude of effects with judgments about their seriousness or importance. Systematic bias consists of non-random distortions in the findings of an assessment method or its component indicators. Failure to account for risk and uncertainty is one of the largest pitfalls of most environmental assessment methods because natural systems are affected by random factors.