Word for the day: “micromort”

Thanks to Bruce Schneier for the heads up on this:

micromort, n.  a numerical score for an event based on a probability of death of 1 in 1,000,000 (1 × 10-6, or 0.0001%).  Unit abbreviation μmt.    Examples: hang-gliding=8μmt, horse-riding=0.5μmt; 100 miles of travel in a car=0.5μmt.

As a blogger with the nom de plume of “Stubborn Mule” put it, “shopping for coffee you would not ask for 0.00025 tons (unless you were naturally irritating), you would ask for 250 grams.” The ability to communicate risks in an accurate but understandable manner is undercut by large denominators expressing very small risks.  Multiplying that fraction to get a micromort makes things more perceptible.


  1. Note that these probabilities are based on findings in the U.K., which may not be equivalent in other regions.
  2. The earliest citation that I could locate was R. A. Howard, “On making life or death decisions” in “Societal Risk Assessment: How Safe is Safe Enough?” (1980, ISBN 0306405547), referenced from Wikipedia.
  3. David Spiegelhalter et al have a wonderful site on the topic of Understanding Uncertainty, with some excellent tools.  Spiegelhalter gave a talk at the LSE’s Department of Economics in 2010 with a very good overview.

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