Safety in Numbers: Dr. Feelsafe (or: How I Learned to Stop Worrying and Love Probability)

This week Safety In Numbers goes on holiday and explores the impact that probability has on our perception of the world.

Safety in Numbers recently returned to the UK from honeymooning in Sri Lanka – a beautiful country populated by wonderful people who cook incredible curry. There are also a lot of elephants (see box).

Elephant population density across Asia

If the top brass at CDR Towers were to broaden the horizons of this blog to cover travel reviews I could happily talk for hours on what a wonderful country it is, but for the purposes of this blog Safety in Numbers will be focusing on one of the perils associated with travelling to southern Asia.

Of course, there are dangers associated with any kind of travel, with a popular soundbite on this being that the majority of road accidents occur within five miles of a motorist’s home1.

However, the perception of this risk – in the face of probability – is diminished by a number of factors, including familiarity, experience and the feeling of control of the situation.

In this regard, a flight to Australia is the polar opposite of driving your car to your local Sainsbury’s for the fifth time this month, and is the reason that people are often much more worried about air travel than they are about any other form of transport. Indeed, according to data published by Northwestern University2, flying is statistically the safest mode of transport, with 0.07 fatalities per 1 billion passenger miles. In other words, you would need to spend 1,700 years travelling non-stop back and forth between London and Sydney on a Boeing 777 before you were likely to be involved in a fatal accident.

In comparison, the chances of dying in a car accident are higher, standing at 7.28 deaths per billion passenger miles, meaning that you’d need to clock up a ‘mere’ 137.4 million miles before being statistically likely to come to any harm.

However, when something bad does happen – as it ultimately will do with the nearly 90,000 commercial flights taking off every day this year3 – it is invariably given blanket news coverage (largely as a result of the improbability of its occurrence). This raises public awareness and heightens the perceived probability of disaster striking.

And nothing gets the hypochondriac’s imagination going like tropical diseases.

Fortunately Sri Lanka is a pretty safe place to visit, health-wise. Earlier this month the World Health Organisation announced that the country had fully eradicated malaria – a particularly incredible achievement given the fact that close neighbour India reportedly suffers around a million cases of the disease a year. There have been no reported cases of another headline-grabbing tourist terror – the Zika virus – and there hasn’t even been a shark attack since 1970.

However, one tropical illness currently causing havoc in Sri Lanka is dengue fever, with the island currently in the throes of one of its worst outbreaks in recent years. According to the Sri Lankan National Dengue Control Unit (NDCU), in the first six months of this year there have already been 23,781 suspected reported cases– nearly double the 16,433 cases reported in the same timeframe last year. With no vaccine and no cure – and a nickname like “breakbone fever” –I started wondering about the probability of contracting the disease whilst on our holiday of a lifetime.

According to the World Health Organization, there are around 390 million dengue infections per year, of which 96 million present with symptoms. With only 25% of cases presenting symptoms, this is already a good start.

Next, it is estimated that 3.9 billion in 128 countries around the world are at risk of infection with dengue viruses, meaning that there is an alarming 10% chance of contracting the disease – though only a 2.5% chance (one in 40) of presenting symptoms. In Sri Lanka, if the number of cases were to continue at their current rate for the remainder of 2016, this would total 58,595 cases for the whole of the year. With a population of more than 20 million, this is a 0.3% chance (one in 347): more good news for the hypochondriac traveller.

Cases of dengue fever by Sri Lankan district per 1,000 people, H1

All of these numbers only really act to placate the overactive imagination, as in practical terms you either contract a virus or you don’t. However, in order to better understand a situation it is far easier to look at it in terms of probability: “what are the chances of X scenario occurring to me?”

This is why every time you open a newspaper you’ll see a new study based on common-or-garden consumer omnibus research. All quantitative research is an attempt to calculate global trends using a non-global pool of respondents – typically 2,000 nationally representative UK adults – so if 20% of respondents say that they have been rained on in any given month that means that the average person can deduce that they have a one in five chance of getting rained in future.

Similarly, a recent poll of Britons on their outlook following Brexit gave cause for optimism, with 59% of people in the UK stating that they believe the country is moving in the right direction – including almost 30% of Remainers.

However, consumer omnibus research can sometimes be a self-fulfilling prophecy, as people will no doubt feel reassured that this newfound confidence is indicative of a rosy future. This is much the same when looking at the probability of a certain outcome; if you’re going camping in a tropical rainforest then you’re going to be more vigilant against mosquitos, sleep under nets and bathe in industrial-strength DEET. So, conversely, this heightened probability can help you plan to adjust your odds of infection and thus reduce your overall risk.

In reality, the dengue fever outbreak in Sri Lanka – while serious – is not a major concern for tourists. In the first half of this year just over one in 1,000 people were affected with it, with a much lower mortality rate. This pales in comparison when you consider that influenza – a relatively familiar disease in the UK – is the fourth largest killer in Sri Lanka, claiming nearly 7,300 lives annually.

So in conclusion, I had a lovely holiday, I managed to avoid catching a disease that I had a very low probability of catching and

Did I catch dengue fever? No
Did it prevent me from having a nice holiday? No
Did probability help me better understand the world around me? Yes
Will I continue to be a hypochondriac on future holidays? Probably.

Written by Chris Jarvis, Head of Research


Research notes

1 Study carried out by Which? Car among 3,800 motorists in August 2009
2 “Comparing the fatality risks in United States transportation across modes and over time”, Ian Savage, Department of Economics and the Transportation Center, Northwestern University, 2011
3 “Aviation Benefits Beyond Borders”, published by The Air Transport Action Group (ATAG) in June 2016. In 2016 it is estimated that there will be 32.8 billion commercial flights worldwide.

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