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Microlives per Micromort Conversion Factor

Source: R/constants.R
MICROLIVES_PER_MICROMORT.Rd

The single conversion factor used throughout this package to translate an acute risk expressed in micromorts into the equivalent chronic risk expressed in microlives.

Usage

MICROLIVES_PER_MICROMORT

Format

A single numeric value: 0.7.

Details

Unit definitions

  • 1 micromort = a 1-in-1,000,000 (1e-6) probability of death from a specific acute exposure.

  • 1 microlife = 30 minutes of life expectancy.

Derivation

Following the life-expectancy framework introduced by Spiegelhalter (2012), a micromort's expected cost in life expectancy ("loss of life expectancy", LLE) is the exposure probability multiplied by the life expectancy remaining at the time of exposure. This package follows the common convention (matching the derivation already used internally by combined_quiz_pairs()) of assuming 40 years of remaining life expectancy for an "average" adult:

LLE_minutes  = 1e-6 * 40 years * 365 days/year * 24 hours/day * 60 min/hour
             = 1e-6 * 21,024,000 minutes
             = 21.024 minutes
microlives   = LLE_minutes / 30 minutes-per-microlife
             = 21.024 / 30
             = 0.7008 -> rounds to 0.7

i.e. MICROLIVES_PER_MICROMORT = (40 * 365 * 24 * 60 * 1e-6) / 30, which evaluates to 0.7008 and is stored rounded to one decimal place as 0.7.

Caveat

The 40-year remaining-life-expectancy assumption is a simplification: the true conversion factor varies by age (a micromort taken late in life converts to fewer microlives than one taken early in life, because there is less remaining life expectancy to lose). 0.7 is therefore a population-average approximation, not an individually-calibrated value. This package has not been able to trace the exact 0.7 figure to a verbatim formula in Spiegelhalter's original BMJ paper (the derivation above is this package's own reconstruction, cross-checked against independent secondary sources that report the same 40-year assumption and arithmetic); the general micromort/microlife framework itself, however, is directly attributable to that paper.

References

Spiegelhalter D (2012). "Using speed of ageing and 'microlives' to communicate the effects of lifetime habits and environment." BMJ 2012;345:e8223. doi:10.1136/bmj.e8223 https://en.wikipedia.org/wiki/Micromort https://en.wikipedia.org/wiki/Microlife