Availability heuristic. But how much will this blog affect you?

Those of us old enough to remember the Ebola outbreak of 1995 will get a shiver down our spine when hearing of that particular disease. It emerged in the town of Yambuku in northern Zaire (now the DRC) and, over the next few weeks, infected over three hundred people in the surrounding area, with two hundred and eighty dying. Continuous news coverage and satellite reporting had recently become common and now terrifying images and dire predictions were being shown to us in every news programme. Coupled with the release of the film Outbreak that year, detailing a similar fever that hops on a flight to the US and starts spreading, and the widely-publicised bestseller The Hot Zone by Richard Preston in the same year, and the message was clear: in an interconnected world with thousands of flights per day, eventually one of these would spread and we would have a pandemic.

Except we didn’t. Even though there are more flights than ever, more people than ever and the areas where these outbreaks naturally occur have more movement of people than ever, it hasn’t happened.

But then in 2013 it, again, seemed it would, as an outbreak in Guinea spread to Sierra Leone and Liberia and killed over 11,000 people over the next three years, prompting a huge WHO response and another round of news-inspired panic, particularly when a contagious, lethal case was diagnosed in the US with others infected. The predictions may have been twenty years late, but it was finally happening.

Again, though, it didn’t. Although you wouldn’t know that from the news coverage.

Once there were a few confirmed cases in the U.K., US, Spain and Brazil, some news networks decided that the pandemic was here, and the news caused parents to keep their children home from school and caused significant stock market drops. The thing is, there was not one case of public transmission in these counties, only a few cases of high-contagious patients infecting primary health care staff, all of whom subsequently recovered. The implications and details are described more fully in Nate Kornell’s article. In the article, he ascribes this to the ‘availability heuristic’, an error of judgement where the mind biases towards information that is easily available rather than what is true.

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A good example of this is trying to think whether there are more words in English beginning with the letter ‘r’ or with ‘r’ as the third letter. Like most people, you probably think there are more words starting with ‘r’ as it’s easier for us to remember these words, but actually, there are roughly double the number with ‘r’ as the third letter (Tversky & Kahnema, 1974). Here we confuse what we personally can recall with reality, and the same happens in diagnosis (here is one paediatrician's discussion on this bias in diagnosing a child with a brain tumour). As well as being a good test to give your friends, this has real importance in diagnosis. Doctors have been observed to diagnoses patients with bacteraemia more frequently if they have recently had a patient with this illness (Poses & Anthony, 1991), and has had serious effects on the prescription of opioids painkillers (see Klein, 2005 for further discussion). The problem is, of course, is how we stop this happening if the problem is really one of how the human mind works.

The key seems to be knowing when to ignore easily available information if it isn’t actually relevant. Firstly it always pays to bear in mind base rates, to try and see each case independently. A difficult process as it goes against our brains instinct to draw links between any information present, but vital in accurate assessment and diagnosis. The second key point seems to be the often difficult but necessary process of second opinions, peer review and feedback as a defence against our own minds intransigence.


Klein JG (2005). Five pitfalls in decisions about diagnosis and prescribing. BMJ : British Medical

Journal, 330:781–783

Redelmeier DA (2005) The Cognitive Psychology of Missed Diagnoses. Ann Intern Med. 142:115-120

Tversky A & Kahneman D (1974) Judgment under uncertainty: heuristics and biases. Science,


Poses RM, Anthony M. Availability, wishful thinking, and physicians' diagnostic judgments

for patients with suspected bacteremia. Med Decis Making 1991;11: 159-68.