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This report has been superseded by some superiodr abnalysis here: May 20 - Improved analysis of impact of Geohashes & GPS Noise I have added comments inline here on points I got wrong. |
Full details to follow, but in brief, I have updated the code here: May 3 - Analysis of Exposure Detection Algorithm to reflect the new intersection algorithms that we plan, as described here: Design for MVP1 HA JSON Changes
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Moving from 20m distance matching to 8 char geohash matching (w/ handling of edges) doesn’t have a very significant impact on false positives or false negatives.
Moving from “any notification” to “At least 66% of points in 30 mins” massively reduces false positives at 150m distance (these mostly arise due to inaccurate GPS)
Moving from a 4 hour exposure notification window to a +/-5 mins notification window also massively reduces false positives at 150m distance
In fact, either one of these measures is sufficient to more-or-less remove false positives at 150m. (we will do further testing to look at false positives at 100m, 50m etc.)
Moving to 7 char geohash tiles undoes all of this… Even with both measures in place, you’ll still get a false positive every 24 hours with 13% 23% of people who are 150m, away from you. Given there willl be a lot of people within this sort of distance it will lead to an unfeasibly high number of false positives.
For a basic “meet for a coffee for an hour” scenario, the biggest threat to getting a match is the risk that one or other party’s phone is not logging due to GPS unreliability issues (SAF-278). Even with the old algorithm, the probability of a match was only 52%.
Reducing the exposure window from 4 hours to 5 mins reduces this to 50%
Requiring 66% of consecutive matches over 30 mins reduces this further to 32%.
If we can fix GPS unreliability (SAF-278), this will be up to 96% (the remaining 4% missing is due to inaccurate GPS (SAF-175) - which we’re also working on).
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