The hazard ranking system is used by the EPA to evaluate the relative risk of hazardous waste facilities. The original system and its newly expanded form have a structured value approach and arrive at a facility's rank through a combination of addition and multiplication of factor scores. The most dangerous sites have the highest scores. This research demonstrates that small increases in data can significantly increase a facility's score if applied to influential system factors. However, the identification of influential factors is difficult. The work shows that factor importance is site specific, that it is, dependent upon the values of other system criteria, and that its evaluation is impeded by the additive/multiplicative structure of the ranking system. Tools for guiding data acquisition are developed to assess generally the influence of two factors, but it is shown that an additive system provides a better method for assessing risk.