Before I discuss this, the attached image contains the overall spreadsheet and most assumptions are detailed on the page. I don’t declare any infallibility (papal or otherwise), but if you think I’ve made a mistake contact me and I’ll upload a corrected version (if required).
I’m not drawing any specific conclusions on what our behaviour should now be, I did this to consider my own risk level for venturing out so that I could do so on a more scientific basis rather than gut reaction.
I used the following assumptions/default values:
- The infection rate is 1 person in 3,900 (the last one i saw)
- You need face to face exposure of 15 minutes to risk transmission of the infection
- Whilst the face covering mitigation %\’s that have been circulating for some time are not proven, I have used heavily moderated values to explore the impact of face coverings
- I\’ve ignored the risks (and maths) of meeting more than 1 carrier of C-19 in any one day
- I have ignored the fact that infection is not evenly distributed around the country, these numbers do not apply to hot spots.
- Equally, these numbers overestimate the risk in cold spots (if that is the opposite of hot spot!)
- This does not consider behaviour patterns whilst out – if you go to the pub and hug all your mates, then your risk likely increases – the behaviour assumed is that social distancing is observed.
- This ignores whether the exposure is indoors or outdoors
- The percentages shown don’t actually demonstrate the chance that you will get a C-19 infection, they show the risk of you participating in a face to face meeting where the risk of transmission can occur. So given transmission is not certain, this means the risk % overstate the chance of you (on average) catching C-19 from that contact.
- Finally, even if the event does transmit C-19 to you, it says nothing about the risks of you falling ill (seriously or not) with C-19
The calculation in the second column is done from this logic:
- The risk of someone being infected is 1/3900 – call this X. In percentage terms this is 0.00000256410256
- The chance of any 1 person you meet being infection free is (1- X) which is 0.99999743589744 or 99. 999743589744%, call this Y
- The chance of all the people you meet being infection free is therefore the value in Y multiplied by itself once for each person, so for 10 people all to be infection free is Y*Y*Y*Y*Y*Y*Y*Y*Y*Y or Y^10. Which is roughly 99.744%.
- Which means the probability of 1 or more people you meet being infect is 100%-99.744% which is my calculated 0.26%.