# What is Availability Bias?

The way we develop our own ideas of the likelihood of an event happening can be greatly influenced by the environment around us. These “subjective probabilities” then form the basis of decisions we make when there is risk involved – but if these probabilities are off, we can end up making decisions that are off the mark.

The concept of availability bias describes how new or recently observed information can skew your perception of an event’s probability. This can occur subtly and surprisingly easily. Imagine your decision whether or not to purchase a lottery ticket. Most tickets have the odds right on them (Winning the pick-6 is 1:1,500,000, as an example), so how we see our chance of winning should be straightforward – it’s 1 in 1,500,000. But, if we’ve just heard about a friend of ours who’s won some money at the lottery, we are likely to think our chance of winning is much higher! Hey, someone we know has had success, so maybe we will too! Because this friend’s story is immediately available to us, we are less likely to remain objective in our assessment of our odds of winning, and more likely to be optimistic!

Or, take a plane ride. Estimates of the likelihood of dying in a plane crash are as low as 1 in 11 million. Yet, imagine how we feel after hearing about a plane crash on the news. Suddenly, flying is very scary – and obviously, then next time we board a plane, if we are thinking about that plane crash on the news, we think the odds of it happening to us are much higher than 1 in 11 million.

Take this article on The Dish about the likelihood of a devastating asteroid hitting the earth. NASA puts the risk of the greatest threat at a likelihood of 1 in 28,000 of making contact with the earth, which is still a fairly small probability. But how do we feel about that risk when we hear the description of the potential impact?

The crater would be about twice the width of Manhattan, and about as deep as the newly constructed Freedom Tower in New York is tall. More than one hundred million cubic meters of rock would be instantly vaporized on impact. The shaking produced would be equivalent of a 7.0 earthquake. If you were standing about 60 miles (100 km) from the impact site, within two minutes you’d be pelted with debris up to about two inches in size. Within five minutes, the air blast generated by the heat of the impact would create hurricane force winds, shattering your windows. If you were standing within about 20 miles away (30 km) – for reference, New York City is roughly 20 miles wide – the effects would be much more serious. The average fragment size headed your way would be about the size of a dishwasher, and within 90 seconds wind speeds would top 500 miles per hour.

Suddenly, the chance of the asteroid hitting seems much more likely than that. Availability bias highlights the details of an unlikely event, make it seem more real, and inflate our perception of the likelihood.

[This post appeared Monday on my blog, Close to the Edge.]