I’ve recently been investigating the work by David Spiegelhalter, a British statistician known for his work relating to risk communication (among other things). He encourages people to think about personal risks in terms of micromorts and microlives.
So… what are micromorts and microlives?
Well, a microprobability is a one in a million chance of something happening. So that gives us a clue.
A micromort is a one in a million chance of sudden death. A micromort is a statistic that can be used when we look at discrete events, and looking at the chance of dying as a result of that event.
It appears that certain “high risk” events such as undergoing general anaesthetic in an emergency operation, skydiving, or riding a motorcycle for 100km each represent approximately 10 micromorts. Interestingly, running a marathon is cited as representing 7 micromorts.
(When we consider a figure like this, it is clearly a generalisation. The exact figure will depend, for example, on your age and physical condition in relation to undergoing a general anaesthetic; the nature of your skydive; and the weather conditions while you are riding your motorcycle. Think of these figures as Bayesian pretest probabilities, which might vary depending on your circumstances.)
A micromort is a good way of quantifying acute risks, or activities that might result in sudden death. Once the activity is over with, however, the clock goes back to zero.
A microlife, on the other hand, relates to chronic risks, which can accumulate over time. A microlife represents an attempt to quantify the impact of undertaking a certain activity or exposing yourself to a certain environment in terms of your life expectancy.
One microlife represents 30 minutes. This is derived from the calculation that 1 million multiplied by 30 minutes represents 57 years, which is approximately the length of an adult lifespan (ie: let’s say adulthood begins after completing your undergraduate study at 22 years, 22 + 57 = 79 years, or somewhere around an OECD country’s life expectancy).
Examples: smoking a cigarette will reduce your life expectancy on average about 15 minutes meaning that each cigarette costs you half of a microlife. Each 5kg you are overweight costs you one microlife per day in terms of life expectancy. On the flip side, the first 20 minutes of moderate exercise you do each day gives you extra microlives.
The micromort and microlife figures are necessarily general and difficult to determine with certainty. But they are a very useful way of conceptualising the acute and chronic risks to which we expose ourselves.