Today I was contemplating the phrase “one in a million” and what things actually are “one in a million”. In going down that rabbit hole, I learned about an interesting unit of measure: the micromort. It’s defined as a one in a million (1 in 1,000,000) risk of death. (Take a look a the linked Wikipedia page for some examples.)
I’m interested in designing good metrics that can make otherwise inscrutable numbers easier to understand and reason about. From that perspective, the micromort is a good start, but still imperfect. My main criticism is that a million (1,000,000) is not a number most people can grasp intuitively.
Instead I would suggest indexing the risk of death relative to some easily understand baseline risk, for example the risk of dying due to any cause (natural or not) during some time period (e.g. a day). Then you could say something like: going sky-diving increases your risk of dying that day by factor of 5.7 (completely made up number). Or in other words, going sky-diving is 5.7 times more risky than going about a completely average day.