“Writing out all of these 9s seems a bit clumsy” Personally, I don’t see it as more clumsy/awkward/inconvenient than trying to learn and accurately use terms like “three nines.” And then you get to situations where it’s a non-integer number of nines (e.g., 0.83 nines): trying to convert that to percentages seems like a pain/intuition block, especially given that most people aren’t familiar with this system. On that point, I would strongly echo the points of AllAmericanBreakfast—whose example of the Richter scale seems like a great example: my impression has been that most (lay) people do not accurately understand these numbers in terms of logarithms, and so it seems like they are less likely to understand this nines system.
Ultimately, I imagine there probably are some mathematically-oriented justifications for using this system, but I think that the key deficiency here is about lay and intuitive understanding, and my impression is that this system does the opposite of helping with that—or at least that it would be far more effective to use better language with existing systems (e.g., saying that the risk has tripled from 0.1% to 0.3% instead of saying the safety has decreased from 99.9% to 99.7%) and/or teach people to better understand the existing systems, rather than introducing a new system.
“Writing out all of these 9s seems a bit clumsy”
Personally, I don’t see it as more clumsy/awkward/inconvenient than trying to learn and accurately use terms like “three nines.” And then you get to situations where it’s a non-integer number of nines (e.g., 0.83 nines): trying to convert that to percentages seems like a pain/intuition block, especially given that most people aren’t familiar with this system. On that point, I would strongly echo the points of AllAmericanBreakfast—whose example of the Richter scale seems like a great example: my impression has been that most (lay) people do not accurately understand these numbers in terms of logarithms, and so it seems like they are less likely to understand this nines system.
Ultimately, I imagine there probably are some mathematically-oriented justifications for using this system, but I think that the key deficiency here is about lay and intuitive understanding, and my impression is that this system does the opposite of helping with that—or at least that it would be far more effective to use better language with existing systems (e.g., saying that the risk has tripled from 0.1% to 0.3% instead of saying the safety has decreased from 99.9% to 99.7%) and/or teach people to better understand the existing systems, rather than introducing a new system.