The power of the MicroMort
Article first published online: 17 APR 2014
© 2014 Royal College of Obstetricians and Gynaecologists
BJOG: An International Journal of Obstetrics & Gynaecology
Volume 121, Issue 6, pages 662–663, May 2014
How to Cite
The power of the MicroMort. BJOG 2014;121:662–663..
- Issue published online: 17 APR 2014
- Article first published online: 17 APR 2014
- Manuscript Accepted: 12 DEC 2013
Audiences tend to giggle nervously when first presented with the term MicroMort because it combines a somewhat flippant name with a deadly association. But this intuitive reaction helps to focus attention on a very useful term that aims to get beyond the usual rather opaque risk statements in terms of person-years or 100 000 population. Its primary purpose is for comparisons, as the absolute size of small risks is tricky to grasp. You can tell people that a 1-in-a-million chance is about the same as the chance of getting 20 heads in a row when flipping a fair coin, but this still does not convey magnitude. To do this adequately we need to use the power of analogy, and not just a single one.
In this issue of BJOG, Walker et al. focus mainly on the analogy of someone of a different age but with the same daily risk of dying as a baby on the day of its birth. Figure 1 shows the daily risk of death, in MicroMorts for men and women in England and Wales in 2011, from which we can immediately read off the author's observation that the daily risk on the first day of life is not reached until the early nineties. A horizontal line at 1 MicroMort/day represents the average daily risk from ‘non-natural causes’ (accidents and violence).
We note in passing that our 7-year-olds have about the lowest mortality of anyone, anywhere, ever, in the history of the human race. But (ignoring the lump of risk-taking youth between 15 and 25 years) from aged 7 the daily dose of MicroMorts, traditionally known as the ‘Force of Mortality’, increases linearly on a log-scale, and therefore exponentially at a rate of 9% per year, and so doubles every 8 years. Gompertz observed this linearity back in the 1820s.
But this is only one analogy, and we can consider a whole lot more for balance. Table 1, based on values in our recent book The Norm Chronicles, shows situations that expose people, on average, to 480 MicroMorts, which the authors consider an average risk on the first day of life.
|Cause of death||Context||Birth equivalents|
|‘Non-natural causes’||England & Wales 2010||16 months|
|All accidents—under 14||England & Wales 2010||2.5 years|
|Driving||UK 2010||160 000 miles|
|Motorcycling||UK 2010||3400 miles|
|Cycling||UK 2010||13 440 miles|
|Travelling by train||UK 2010||3 600 000 miles|
|Travelling by light aircraft||USA 1992–2011||7200 miles|
|Travelling by passenger jet||USA 2010||2 900 000 miles|
|Anaesthesia for non-emergency operation||UK||48 operations|
|Serving in Afghanistan (peak risk period)|| |
All UK forces
|Flying in Bomber Command in WW2||Royal Air Force 1939–1945||8 minutes|
|Scuba diving||UK 1998–2009||96 dives|
|Running marathons||USA 1975–2004||70 runs|
|Base-jumping||Kjerag Massif, Norway||1 jump|
|Personal risk from asteroid||34 000 years|
The Table deliberately shows a wide range of activities to try and avoid undue influence of ‘framing’. Single analogies, such as driving 160 000 miles, will tend to minimise the apparent risk if driving is perceived as being very safe, whereas others, such as serving 12 days in Afghanistan or hurling yourself off a rock in Norway, may tend to exaggerate the risk due to the extreme negative connotations. Therefore a mixed selection may be appropriate.
We note that we do not include chronic risks, such as cigarette smoking, obesity or pork pies. These are unlikely to kill you on the spot (unless you choke on the pork pie) and so their impact is better expressed as a change in lifelong-risks, such as reduction in life-expectancy, additional risk of early death, or increase in annual risk of death. For example, smoking 20 a day roughly doubles your daily MicroMorts, or equivalently is expected to take about 8 years off your life. Due to Gompertz's observation, we can also consider the smoking as essentially adding 8 years to your ‘effective age’, and this is independent of your current age.
The type of comparisons shown in Table 1 can be controversial. When the government drugs advisor Professor David Nutt rather mischievously compared the risks of ecstasy with the addiction to horse-riding, which he termed equasy, and showed they were of roughly equivalent harm, this was greeted with political uproar and his services were eventual dispensed with. But why the fuss? Illegal drugs and horse-riding are both leisure activities, voluntarily undertaken by young people for fun. But one is wholesome and politically acceptable, and the other is not.
Why are we trying to communicate risk anyway? It needs to be considered whether we are trying to increase understanding, change behaviour, or encourage shared-care decisions based on an informed choice. My favourite objective was explored by psychologists who were trying to encourage ‘immunity to misleading anecdote’, in which they showed in a randomised trial that transparent risk communication led to participants being less influenced by unrepresentative stories of the type frequently found in families, media and the internet–someone who smoked all their life and lived to 90, drank carrot juice and their cancer went away and so on. These stories may be true, and may reflect ‘possibilities’, but they do not reflect probabilities.
Of course all of these comparisons can only be rough assessments—they are only averages and do not reflect individual circumstances. But they provide ballpark figures to start important conversations, and if they use story-telling techniques of analogy and emotional response, without seeking to unduly manipulate behaviour, then these ideas should be welcomed.
Disclosure of interests
Nothing to disclose.
- 1Office for National Statistics. UK Interim Life Tables, 1980–82 to 2008–10 [Internet]. Office of National Statistics, 2011 [http://www.ons.gov.uk/ons/rel/lifetables/interim-life-tables/2008-2010/sum-ilt-2008-10.html]. Accessed 30 October 2012.
- 2On the nature of the function expressive of the law of human mortality, and on a new mode of determining the value of life contingencies. Phil Trans R Soc Lond 1825;115:513–585..
- 3The Norm Chronicles: Stories and Numbers about Danger. London: Profile Books, 2013., .