Last night I went to the cinema to see 127 Hours – a movie about a young man who had to go to extraordinary and frightening lengths to save himself while he was canyoneering (or canyoning to those outside the US) in Utah.
I had the privilege of hiking in Utah a few years back. While I enjoy the outdoors, I don’t particular enjoy activities I view as high risk – canyoneering, rock climbing, pot holing etc. My partner Laura, on the other hand, loves these activities, particularly rock climbing, and is consistently encouraging me to join her. After watching 127 Hours, it got me thinking; how worried should I be every time Laura goes rock climbing?
Pick up any newspaper or read an online site and you are constantly bombarded with statistics. A quick search online on the risk of death from rock climbing and the risk is quoted as the number of deaths per year. The American Alpine Club, in its yearly compendium Accidents in North American Mountaineering, reported 15 fatalities in the United States in all of 2007, and a yearly average of 25 deaths. But how do we compare year on year? For example, in the last 50 years, people have typically spent more time on leisure activities; the number of rock climbers has, well, climbed. So measuring ‘deaths per year’ does not tell us how dangerous an activity is: instead it combines how risky it is with how many people do it – and combines them in such a way that it is impossible to disentangle the two. This way of measuring risk doesn’t take into account the number of people climbing, or how often they are climbing, so absolute comparisons become difficult.
For other causes of death, such as homicide, we often read about risk in “death per million”. For example, the homicide rate in the UK is currently 14 per million per year while in the USA it is just over three times that at 50 per million per year. This takes into account the size of the population and allows us to make comparisons across different years as population changes. Where the total population is known - the population of a country, the number of rock climbers, etc - it is possible to calculate this statistic. For example, in the USA, historically the deaths per million per year from falling (in the home, not from rocks) and from drowning were approximately 62 and 36 respectively.
However, these statistics can be misleading if a reader tries to draw comparisons between different risks because the underlying metric once again hides risk. The measurement “death per million” conceals hides a number of factors, most notably, that risk is not distributed uniformly and the age of those that die.
A measure called ‘loss of life expectancy’ addresses this second issue. Broadly speaking, this measures the additional life expectancy of a person if a given risk is eliminated. For example, if one were to eliminate motor vehicle accidents, we could expect to live an additional 204 days. This metric also allows us to look at how activities, such as having an air bag in a car, can add days to your life if their use was mandatory. We also find that when we use this metric, the loss of life expectancy for drowning is actually greater than falling – intuitively, this is explained as fatal falls typically occurring in the elderly, while many children are the victims of drowning.
However, this metric still does not give us an intuitive measure of how risky an event is to us; it only tells us how one activity compares to another. Nor do any of these metrics give us a method of distinguishing between those who rock climb once a week and those who climb every day.
The UK Health and Safety Executive (HSE), using an estimate of the number of people who climb and the intensity of the activity, estimate that the risk of a fatal accident is 1 in every 320,000 climbs. This seems to be nearer to what we are looking for. By measuring it in the number of climbs, we take into account how intense a climber a person is. According to Laura, at 5 climbs per trip and 20 visits per year, her yearly risk of fatal accident is 1 in 8,000. Using HSE statistics, this is similar to the risk from approximately 24 scuba dives, 100 canoeing trips or one pregnancy.
All of the above metrics have strengths and weaknesses. Some allow us to compare events per year; some allow us to measure the positive effect of activities; and some allow us to measure the risk to an individual. Every day we hear statistics quoted to us and different activities/options compared. When defending/criticising these statistics, people often look at how the data is collected or interpreted. Maybe the metric - the way we measure - should be subjected to the same scrutiny.