Modifying Social Vulnerability Indices to Complement Physical Exposure Risk Analyses

Multivariate social vulnerability indices, used to compare communities’ susceptibility to adverse disaster outcomes, cannot be combined with physical exposure risk estimates without double-counting some effects and potentially obscuring risk sources. Further, the techniques used to create the indices appear to have methodological concerns, including diluting the signal of poverty in defining overburdened communities, creating conflict with some federal and state policy definitions. This research examined ways to modify social vulnerability indices to isolate the residual social risk that could serve as a multiplier on flood or other quantitative risk assessments, and thereby create an equity-weighted risk metric. We advanced this concept by reviewing the social vulnerability studies that tested the explanatory power of sociodemographic indicators on disaster outcomes, when controlling for physical exposure risk. We then suggested a new type of vulnerability index, or Equity (E) Index, that isolated the indicators with the strongest evidence that they represented residual social risk and examined their potential magnitude as multipliers on physical exposure risk. We also used multiple decision science techniques to minimize some undesirable index construction issues. Whereas the literature provided mixed evidence for all indicators, poverty, disability, and old age had the highest confidence that they amplified physical risk. Other factors such as race, ethnicity, acculturation, mobility limitations, children under five, and female single parents had an intermediate level of support. Indicators of median home value and low education were less frequently tested, and gender and percentage of renters had ambiguous signs. An initial exploration of empirical models suggested that, on average, the E index approximately doubled the effect of physical harm when assessing outcomes for the most vulnerable groups, but further research and validation are required.