Cwa-documentation: Risk Calculation needs Clarification and Summary

[cougarten]

There are various risk values of some form. There is no central place to look them up.

Use-case: I got a warning about a "low risk" encounter. I want to know what kind of encounter counts as "low risk".

Existing important Risk Values:

• Risk Level
• Total Risk Score
• Combined Risk Score

Risk Level

Missing Summary

This is the user facing warning when an exposure happened.

• possible values: "low risk" or "high risk"
• based on the "combined risk score" for the overall summary (?)
• based on the "total risk score" for individual encounters (?)
• mapped according to the following parameters: https://github.com/corona-warn-app/cwa-server/blob/master/services/distribution/src/main/resources/master-config/risk-score-classification.yaml
• covering the last 14 days
• examples
  
  
  
  • [low risk, minimum requirement]
  
  
  • [high risk, minimum requirement]
  
  

Clarification Needed:

• The app displays two values, one total risk as a title, one as "n exposure(s) with low/high risk". Is the latter based on the "total risk score"?

Combined Risk Score

Missing Summary

Total risk deducted from all past encounters with known covid positive persons.

• Based on Total Risk Scores
• possible value: ?

Total Risk Score

Missing Summary

Risk of infection for a past single encounter with a known covid positive person.

• possible value: 0-4096
• based on
  
  
  
  • Transmission risk level of person tested positive (https://github.com/corona-warn-app/cwa-documentation/blob/master/cwa-risk-assessment.md)
  
  
  • context of the encounter (proximity, duration, when; https://github.com/corona-warn-app/cwa-documentation/blob/master/solution_architecture.md#risk-score-calculation)
  
  

Clarification Needed:

Dismissal of short or distant encounters

According to the transmission risk level calculation short or distant encounters are dismissed. The exposure config however still has bins for these values. It is not clear what a brief and/or distant encounter with a known covid positive person will result in.

Possible would be:

• No exposure warning: the encounter was dismissed
• Risk Level "Low risk": the encounter was noted and likely rated with a Combined Risk Score of 0, which maps to "low risk"

What is the value of the minimalRiskScore?

According to the docs there is another threshold at play. I can't see what it's set to or if it remains unset. Here the relevant paragraph:

Google/Apple framework allows to set a minimalRiskScore to exclude exposure incidents with scores lower than the value of this property. In the current version of the API the time spent within the ranges of attenuation buckets are accumulated over all exposure incidents during one matching session. Since the number of requests is currently limited, it is not possible to get these values for each day and each exposure incident separately. While by default there is no minimum value set, this value is being configured accordingly, so that presumably irrelevant exposure incidents are excluded.

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Internal Tracking ID: EXPOSUREAPP-3495

[heinezen]

Hey @cougarten ,

Thanks for the input. I have forwarded your feedback to the dev team by creating an internal Jira ticket (EXPOSUREAPP-3495). They will now investigate how the documentation can be reformulated or extended to make it more understandable. We will inform you about updates on the progress of the rewrite here in this issue.

Regards,
CH

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Corona-Warn-App Open Source Team

[hoehleatsu]

The value of the minimum risk score is 11 as linked at the end of CWA Risk Assessment document.
Maybe the following blog post can help answer some of your questions?

[MikeMcC399]

@cougarten
https://www.coronawarn.app/en/blog/2020-12-30-cwa-behind-the-scenes/ and
https://www.coronawarn.app/en/blog/2020-12-17-risk-calculation-exposure-notification-framework-2-0/

provide a lot of updated information about risk calculation. Does this answer your question now?