lies and statistics, statistics

Lies, Damned Lies, and Statistics (39): Availability Bias

availability bias on newspaper frontpage

example of availability bias on a newspaper’s frontpage

(source)

This is actually only about one type of availability bias: if a certain percentage of your friends are computer programmers or have red hair, you may conclude that the same percentage of a total population are computer programmers or have red hair. You’re not working with a random and representative sample – perhaps you like computer programmers or you are attracted to people with red hair – so you make do with the sample that you have, the one that is immediately available, and you extrapolate on the basis of that.

Most of the time you’re wrong to do so – as in the examples above. In some cases, however, it may be a useful shortcut that allows you to avoid the hard work of establishing a random and representative sample and gathering information from it. If you use a sample that’s not strictly random but also not biased by your own strong preferences such as friendship or attraction, it may give reasonably adequate information on the total population. If you have a reasonably large number of friends and if you couldn’t care less about their hair color, then it may be OK to use your friends as a proxy of a random sample and extrapolate the rates of each hair color to the total population.

The problem is the following: because the use of available samples is sometimes OK, we are perhaps fooled into thinking that they are OK even when they’re not. And then we come up with arguments like:

  • Smoking can’t be all that bad. I know a lot of smokers who have lived long and healthy lives.
  • It’s better to avoid groups of young black men at night, because I know a number of people who have been attacked by young black men (and I’ll forget that I’ll hardly ever hear of people not having been attacked).
  • Cats must have a special ability to fall from great heights and survive, because I’ve seen a lot of press reports about such events (and I forget that I’ll rarely read a report about a cat falling and dying).
  • Violent criminals should be locked up for life because I’m always reading newspaper articles about re-offenders (again, very unlikely that I’ll read anything about non-re-offenders).

As is clear from some of the examples above, availability bias can sometimes have consequences for human rights: it can foster racial bias, it can lead to “tough on crime” policies, etc.

More posts in this series are here.

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One thought on “Lies, Damned Lies, and Statistics (39): Availability Bias

  1. “It’s better to avoid groups of young black men at night, because I know a number of people who have been attacked by young black men (and I’ll forget that I’ll hardly ever hear of people not having been attacked).”
    I was born and raised in Southern Africa where I raised two “young black men” who were role models in their communities. There were issues about violence in that part of the land that made me fear for their safety. We came to America and everyday I feared for their interaction with the police because a young black man is “typically a criminal”, he is the person typically arrested without probable cause, he is framed for murder and as we saw in Troy Davis’ Case, he can be killed by the state in spite of doubtful evidence.
    Sadly the stereotyping and framing and deaths of innocent black men still permeates society.

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