How is ExposureConfiguration set in CoronaWarn ?

[pdehaye]

The CoronaRiskScoring project is trying to assess how different contact tracing apps are being parametrized. We are first focusing on apps using the Apple API as it will enable quickest comparison.

Project considered
We are looking at CoronaWarn.

What we need
We need pointers to the places in the code where the following parameters are set:

• ENExposureConfiguration.attenuationLevelValues,
• ENExposureConfiguration.daysSinceLastExposureLevelValues,
• ENExposureConfiguration.durationLevelValues,
• ENExposureConfiguration.transmissionRiskLevelValues and
• ENExposureConfiguration.metadata.attenuationDurationThresholds

Nice to have
We would also like to understand what is done with ENExposureDetectionSummary and ENExposureInfo, after the risks are calculated.

Excellent to have
This is only the beginning for us. We want the above so we can track how the parameters are being changed. It is likely actually that this is passed dynamically to the apps. If so, we need to know the process behind these changes, so we can ask the right people for this information.

If you have pointers in the right direction, they would be greatly appreciated.

[sebastianpicklum]

The quickest way for all Apple API apps would be simply searching for the method names in the repository.

CoronaWarn's configuration is dynamic, so that the app can be updated with new parameters when there are newer insights available.

See private extension ENExposureConfiguration in https://github.com/corona-warn-app/cwa-app-ios/blob/development/src/xcode/ENA/ENA/Source/Client/HTTP%20Client/HTTPClient.swift

[pdehaye]

See also corona-warn-app/cwa-documentation#312

[pdehaye]

The code here points to

https://svc90.main.px.t-online.de/version/v1/configuration/country/DE/app_config

[pdehaye]

Quite a bit of information about this now consolidated here:

• Exposure Configuration
• Attenuation & Duration Configuration
• App Configuration, including minimum risk score
• Risk Score Classification

[corneliusroemer]

The documentation for how risk is calculated has been improved significantly, in particular with an example.
https://github.com/corona-warn-app/cwa-documentation/blob/master/cwa-risk-assessment.md

A few things are still unclear to me, however:

• How is duration calculated? E.g. when other phone spotted at 0, 2, 9, 13, 21, 35, 54min? What's the total duration?
• How are distance averages calculated - e.g. if the above encounters alternate between -30 and -80 dB?
• The "risk normalisation" still confuses me, what's the purpose? To decrease importance of many low risk encounters vs one high risk one?

Maybe these questions are answered at the level of GAEN, still would be good to know and explain to the interested public. I can see quite some obvious sub-optimality there in the choice of algorithm/parameters - but can only really talk about it for sure once I know the exact algorithmic details.