With advances in technologies like cancer screening, we need to be as clear as possible when stating results in terms of probabilities

It’s not just patients who sometimes find risks and probabilities difficult to understand. Doctors can be challenged by them too.

In an experiment in 2004, psychologist Professor Gigerenzer and colleagues at the Max Planck Institute for Human Development set a group of experienced doctors the following problem:

About 1% of women have breast cancer and a cancer screening method can detect 80% of genuine cancers but also has a false positive (or false alarm) rate of 10%.  What is the probability that women whose test produces a positive result actually do have breast cancer? Most of the doctors thought it was 70%.

Another set of doctors were asked the same question, but this time they were given the data as whole numbers. They were told that 10 in every 1,000 women have breast cancer and that, of these 10, 8 will give a positive screening result…while of the 990 who do not have cancer, 99 will produce a false positive result. Asked to estimate the probability that women with a positive result have cancer, most of the doctors could see that it was 8/(99+8), so roughly 7.5 %.

Changing raw probabilities into hard numbers helps makes things much clearer.  Indeed, simply changing 8% of people to 80 people out of every 1,000 can make a big difference in understanding.

Knowing how the human mind computes clearly helps when it comes to deciding how best to communicate probability. Percentages depend on your base figure and context doesn’t always make the base figure clear.  The same information presented as ‘counting heads’ clearly spells out the group being referred to at each stage and so avoids the problem.

So chances are we do all get probability, it’s how it is explained and communicated that matters most. And when it comes to worrying about having or not having serious conditions, it can make the difference between reassurance or added anxiety.

See the Understanding Uncertainty site for a great animation on screening.

Print PDF