The precision and accuracy of U.S. environmental impact statements.

A post-project audit was conducted of 239 impacts forecast in an representative cross-section of 29 U.S. environmental impact statements. Results must be qualified because of the imprecision of EIS forecasts and the quality of data available to a retrospective evaluator. Relatively few forecasts were found to be inaccurate and even fewer unanticipated impacts were identified. However, only 30% of the impacts were unqualifiedly close to their forecasts, with almost as many rated accurate principally by virtue of the vagueness of the forecasts.