Exposure Risk Value Calculation in ExposureNotification Version 1
Learn how to determine a user’s risk of exposure.
Overview
The ExposureNotification framework uses an Exposure Risk Value to assist the user in determining whether they may have been exposed to the coronavirus. Health authorities have flexibility in calculating this value by setting weights and values in a configuration object.
To adopt this method of risk assessment, set the ENAPIVersion key to the integer value 1 in the app’s Info.plist.
The following diagram illustrates the data structure and formula used to calculate the Exposure Risk Value.
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The following parameters are used to calculate a risk for each exposure incident:
- Transmission Risk
Transmission risk is intended to reflect the status of infection in the affected user and its effect on risk of transmission. The value is based on the affected user’s symptoms, when symptoms first appeared, level of diagnosis verification, or other determination from the app or a health authority.
- Duration
Cumulative duration of the exposure. The framework measures this value.
- Days
Days since the exposure incident. The framework measures this value.
- Attenuation
The attenuation (
transmission power - RSSI) can vary during an exposure event. Attenuation values greater than 0 are weighted by the duration at each risk level and averaged for the overall duration. The framework measures and calculates this value.
The totalRiskScore is calculated using the following formula:
totalRiskScore = transmissionRiskValue * durationRiskValue *
daysSinceLastExposureRiskValue * attenuationRiskValueLevel values are in the range of 0-8. While the formula’s range can be up to 4096, the framework limits totalRiskScore to the upper limit of ENRiskScore, which has a maximum value of 255.
The following diagram illustrates an example of an Exposure Risk Value calculated for a person who was exposed to another person who was diagnosed positive.
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For this example, the other user reported a positive diagnosis, the encounter between the two devices lasted 14 minutes, it happened 4 days ago, and the signal strength attenuation between their phones had a weighted average of 68. The totalRiskScore is limited and set to 255.
To exclude exposure incidents with a risk score below a certain amount, set minimumRiskScoreFullRange. The framework doesn’t use this property when calculating matchedKeyCount or daysSinceLastExposureThreshold.
Topics
Exposure Information
Level Configuration
attenuationDurationThresholdsattenuationLevelValuesdaysSinceLastExposureLevelValuesdurationLevelValuestransmissionRiskLevelValuesmetadata