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Merge pull request #200 from corona-warn-app/SabineLoss-patch-2

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@ -208,17 +208,17 @@ The information listed above is not visible to the user, but is used internally
![Figure 12: Risk calculation](images/solution_architecture/figure_12.svg "Figure 12: Risk calculation") ![Figure 12: Risk calculation](images/solution_architecture/figure_12.svg "Figure 12: Risk calculation")
*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". *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 predefined 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 the risk scores is used as the **total risk score** of the individual exposure. Each of those value ranges is 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 the risk scores is used as the **total risk score** of the individual exposure.
In order to incorporate the time spent within the ranges of attenuation buckets mentioned before, each of those three buckets is assigned a weight value, as shown in *Figure 13*. In order to incorporate the time spent within the ranges of attenuation buckets mentioned before, each of those three buckets is assigned a weight value as shown in *Figure 13*.
The individual time values are multiplied with their according weight (*weight_1*, *weight_2* and *weight_3*). The individual time values are multiplied with their according weight (*weight_1*, *weight_2* and *weight_3*).
Their sum and a default bucket offset (called *weight_4* in *Figure 13*) forms the *Exposure Score*. Their sum and a default bucket offset (called *weight_4* in *Figure 13*) forms the *Exposure Score*.
The *total risk score*, provided by the Exposure Notification framework is then normalized and multiplied with the exposure score, resulting in the combined risk score. The *total risk score* provided by the Exposure Notification Framework is then normalized and multiplied with the exposure score, resulting in the combined risk score.
This combined risk score 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. The combined risk score 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 the values above are multiplied with each other, a single category with a risk score of 0 means that also the overall risk score will be 0. As the values above are multiplied with each other, a single category with a risk score of 0 means that the overall risk score is also 0.
Additionally, a central threshold for the combined risk score specifies, whether an exposure event should be considered or not. Additionally, a central threshold for the combined risk score specifies whether an exposure event should be considered or not.
![Figure 13: Calculation of the combined risk score](images/solution_architecture/figure_13.svg "Figure 13: Calculation of the combined risk score") ![Figure 13: Calculation of the combined risk score](images/solution_architecture/figure_13.svg "Figure 13: Calculation of the combined risk score")