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Update risk score calculation 

follow documentation at https://developer.apple.com/documentation/exposurenotification/enexposureconfiguration

fix #94
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Thomas Klingbeil 2020-05-19 12:43:08 +02:00
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images/solution_architecture/figure_12.svg
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solution_architecture.md
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@ -187,7 +187,9 @@ The information listed above is not visible to the user, but is used internally
![Figure 12: Calculation of the risk](images/solution_architecture/figure_12.svg "Figure 12: Calculation of the risk")
*Figure 12* displays how the total risk score is being calculated. The application is provided with a set of parameters, which are marked in blue within the figure. Each risk category (days since exposure, exposure duration, signal attenuation and the transmission risk factor) receives an input value from the event, which is then mapped to a pre-defined value range. Each of those value ranges gets assigned a risk score from 1-8, where 1 represents a very low risk and 8 represents are very high risk. This means that from each of the rows in the figure, one value is selected according to the input value for the corresponding category. The resulting value is multiplied with a weight factor, allowing to define which category has a higher impact on the overall risk score. The sum of those weighted values can then be used to determine, which defined risk level should be displayed to the user (e.g. “low risk” or “high risk”). For this decision, app-defined thresholds for the individual risk levels apply. Additionally, a central risk threshold specifies, when the user shall be explicitly notified about an exposure event.
*Figure 12* displays how the total risk score is being calculated. The application is provided with a set of parameters, which are marked in blue within the figure. Each risk category (days since exposure, exposure duration, weighted signal attenuation and the transmission risk factor) receives an input value from the event, which is then mapped to a pre-defined value range. According to the [documentation of the framework](https://developer.apple.com/documentation/exposurenotification/enexposureconfiguration), "the attenuation is weighted by the duration at each risk level and averaged for the overall duration".
Each of those value ranges gets assigned a risk score from 0-8, where 0 represents a very low risk and 8 represents a very high risk. This means that from each of the rows in the figure, one value is selected according to the input value for the corresponding category. The product of risk scores is the total risk score, which is used to determine, which defined risk level should be displayed to the user (e.g. “low risk” or “high risk”). For this decision, app-defined thresholds for the individual risk levels apply.
As all values are multiplied, a single category with a risk score of 0 means that also the overall risk score will be 0. Additionally, a central risk threshold specifies, when the user shall be explicitly notified about an exposure event.
Note that the transmission risk level plays a special role: It can be defined by the app and be associated with each individual diagnosis key (i.e. specific for each day of an infected person) that is being sent to the server. It contains a value from 1 to 8, which can be used to represent a calculated risk defined by the health authority.