FEMA’s National Risk Index (NRI) dataset provides information for communities most at risk to 18 different natural hazards. To calculate risk, the NRI uses three inputs; expected annual loss (EAL), social vulnerability, and community resilience. EAL measures the expected loss of building value, population, and agriculture value each year due to natural hazards. Social Vulnerability is a consequence enhancing component and analyzes demographic characteristics to measure the susceptibility of social groups to the adverse impacts of natural hazards. Community Resilience is a consequence reduction component and uses demographic characteristics to measure a community’s ability to prepare for, adapt to, withstand, and recover from the effects of natural hazards. This function of these inputs can be visualized as Risk = EAL * (Social Vulnerability / Community Resilience).
The NRI provides three outputs for EAL and risk: values, scores, and ratings:
Values: Values are the absolute basis for Risk and EAL and are measured in units of dollars, representing a community’s average economic loss from natural hazards each year. This is not included in PolicyMap, as values are calculated independently for each indicator, therefore the range and actual meaning for each value varies by indicator. For example, a census tract may have a risk value of 10,000 for one hazard and be rated as very high risk, whereas the risk value for another hazard may be 10,000 and be rated as relatively moderate risk. The data dev team wanted to avoid confusing users with the values, and to note they are not included in FEMA’s map either.
Scores: Scores represent the national percentile ranking of a community’s EAL/risk value compared to all other communities at the same level (county or Census tract). Again, they are not comparable across indicators, meaning a risk score of 55 for one hazard does not mean it has the same risk as a score of 55 for another hazard. Rather, this means that this county/census tract has higher risk than 55% of the rest of the country. Scores are located inside the info bubble for a geography.
Ratings: Ratings provide the most direct understanding to a community’s risk. They are provided in one of five qualitative categories describing the community’s component value in comparison to all other communities at the same level and range from “Very Low” to “Very High”. Rating categories are specific to hazards, therefore the range of risk scores that fall within each rating category is not the same for every indicator. For example, a risk score of 25 (i.e., 25th percentile) may be considered moderately low for one hazard but considered very low for another hazard. FEMA determined ratings using an unsupervised machine learning technique known as k-means clustering, which were applied to each hazard.