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Understanding Scores and Ratings

The National Risk Index provides relative Risk Index scores and ratings based on data for Expected Annual Loss due to natural hazards, Social Vulnerability, and Community Resilience. Separate scores and ratings are also provided for each component: Expected Annual Loss, Social Vulnerability, and Community Resilience. For the Risk Index and Expected Annual Loss, scores and ratings can be viewed as a composite score for all hazards or individually for each of the 18 hazard types. You can explore all scores and ratings using the interactive National Risk Index map.

Scores

A community’s score describes its relative position among all other communities at the same level for a given component. All scores are constrained to range of 0 (lowest possible value) to 100 (highest possible value). To achieve this range, the values of each National Risk Index component are rescaled using min-max normalization, which preserves their distribution while making them easier to understand. For example, if the minimum value is 18 and the maximum is 2,500, then the minimum becomes 0, maximum becomes 100, and all values between them are transformed proportionally to fit in the 0 to 100 range.

For Expected Annual Loss values specifically, a cube root transformation is applied before min-max normalization. Without the transformation, Expected Annual Loss values are heavily skewed by an extreme range of population and building value densities between urban and rural communities. By applying cube root transformation, the National Risk Index controls for this characteristic and provides scores with greater differentiation and usefulness.

Ratings

For every score there is a qualitative rating that describes the nature of a community’s score in comparison to all other communities at the same level, ranging from “Very Low” to “Very High.” Because all ratings are relative, there are no specific numeric values that determine the rating. For example, a community’s Risk Index score could be 8.9 with a rating of “Relatively Low,” but its Social Vulnerability score may be 11.3 with a rating of “Very Low.” The rating is intended to classify a community in relation to all other communities for a specific Risk Index component.

To determine ratings, a methodology known as k-means clustering or natural breaks is applied to each score. This approach divides all communities into groups such that the communities within each group are as similar as possible (minimized variance) while the groups are as different as possible (maximized variance).

To assist with mapping and visualization, standard color schemes have been applied to the qualitative ratings. Risk Index ratings are represented using a diverging blue (Very Low) to red (Very High) color scheme. Expected Annual Loss, Social Vulnerability, and Community Resilience ratings are represented using sequential color schemes (single color at various intensities). According to the National Risk Index, higher Expected Annual Loss, higher Social Vulnerability, and/or lower Community Resilience increase your overall risk. Darker shading in the map layers represents a higher contribution to overall risk. When source data are not available or a score cannot be calculated, then additional ratings are used and shown in white or shades of gray.

Four rating legends for National Risk Index, Expected Annual Loss, Social Vulnerability, and Community Resilience. National Risk Index ratings are dark red (very high), red (relatively high), yellow (relatively moderate), blue (relatively low), dark blue (very low), white (no rating), light gray (not applicable), and dark gray (insufficient data). Expected Annual Loss, Social Vulnerability, and Community Resilience (orange,  green, and purple, respectively) have shaded levels of risk as mentioned in the body text. Expected Annual Loss also has white (no expected annual losses), light gray (not applicable), and dark gray (insufficient data). Social Vulnerability and Community Resilience both have gray (data unavailable).

Zero or Null Expected Annual Loss Values

Zero or null values for any of the Expected Annual Loss factors will be displayed differently based on the reason for those values.

A county or Census tract in which the Expected Annual Loss is zero either has no building value, population, or agriculture value exposed to the hazard, or has a calculated annualized frequency of zero. (However, some hazard types apply a minimum annual frequency in these cases.) These areas are displayed in the National Risk Index application as having “No Expected Annual Loss” for the designated hazard.

To distinguish between areas where no risk for a hazard type exists and those where the hazard type is not deemed possible, a control table was generated to designate which counties have some probability of being impacted by a hazard event. For example, Coastal Flooding Expected Annual Loss is not computed for inland areas. These areas are displayed in the National Risk Index application as “Not Applicable” for Expected Annual Loss computation for the designated hazard type.

If a component used to calculate the Expected Annual Loss of a Census tract or county for a hazard type has a null value, the National Risk Index informs the user that there is insufficient data to calculate the Expected Annual Loss. Certain hazard types, such as Wildfire, Lightning, and Landslide, only have source data for the conterminous United States, meaning that both Alaska and Hawaii do not have enough data to determine Expected Annual Loss. These areas are displayed in the National Risk Index as having “Insufficient Data” to compute the Expected Annual Loss for the designated hazard.

Please visit each risk component’s or risk factor’s page to learn about specific scores, ratings, and source data.

For comprehensive details about National Risk Index scores and ratings, see the National Risk Index Technical Documentation.