University of Bristol
Wellcome Trust
Recommended by:
Society of Biology
PEEP for Physics & Ethics at GCSE

Probability and Chance

A collection of dice with many sides.Will it happen to me ?

Humans are not great at calculating the probability of risks. In fact we prefer to judge by our personal experience or belief.

If a coin lands on heads five times in a row most people will believe that it is more likely to land on tails on the next throw.

You can test this on yourself by  clicking here to predict a series of coin tosses.

No matter how many times a coin lands heads in a row, the probability that it will land on tails the next time is one in two (0.5).

Which do you think is more probable in the UK, winning the lottery or being killed in a road accident? There is roughly a one in fourteen million of the former and one in four million of the latter. Which will you stop – buying a lottery ticket or going in cars?

People put spin on what they read to overestimate the chance of something good happening to them and underestimate chances of something bad.

If you were told that you have a one in fourteen million chance of getting cancer in the next seven days people will say 'oh well it is obviously not going to happen to me it is so infinitesimal' but the fact that there is a one in fourteen million chance of winning the lottery people think 'yes, it's got to be someone why can't it be me.

Mark Griffiths of Nottingham Trent University

What about a risk that you have little choice about compared to a voluntary risk? There is some evidence that people tend to over-estimate the risks of passive smoking, while smokers underestimate the risk of their own smoking. Read more about people and health risks here.

How serious a risk is it?

Risk assessment is the identification, quantification, and evaluation of risks. Its aim is to arrive at the best understanding possible, assessing the probability of the event occurring and the seriousness of the outcome.

Whether it’s your money, a local epidemic or global climate change we all believe that the more serious the outcome, the less acceptable the risk.

Risk assessment involves weighing up chance and uncertainty.

You can read more about coincidence and chance in an
 article by Andrew Stickland



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Read this story from the BBC “Radon gas linked to cancer deaths” about risks of radon gas building up in the home. Then do the two polls below. 


Now assume you want to buy a house in Cornwall in an area known to be high in radon – how much does this story worry you?
Lots and lots
A bit
Not at all

People are known to downplay risks that result from voluntary choice: driving, smoking, scuba diving etc and overemphasise risks that arise from events they have little or no choice about. This is thought to be linked to a sense of controllability whereby less risk is perceived in situations that are under personal control.

Remember perceived control is not always the same as actual control – people think they are more likely to win the lottery if they pick their numbers as opposed to opting for a ‘lucky dip’ where they are allocated at random.