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.Ask a Scientist:
Freeman Dyson, retired Professor at the
Princeton Institute for Advanced Study.


Do you feel that modern scientists have an ethical role to play in the world?

Very definitely. Of course we don't all agree about ethics, but still the ethics of science I think is extremely important to most of us. First of all, not claiming to know more than you know. That's the most important thing, that science is about uncertainty rather than about certainty.

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So how do we calculate uncertainty?

The probability of a certain event is 1. This is equivalent to you being 100% sure it will happen.

With a coin toss it will fall as either heads or tails, on tossing a coin repeatedly over time this will average out so that half the time, that’s 1 in 2, it will fall as heads.

  • A probability of 1 in 2 = ½ = 0.5 = 50%

Scientists make predictions about outcomes of events according to their calculated probability level. A U.S. Environmental Protection Agency report written in 1995 by Titus and Narayanan concludes that that there is

  • a 90 percent chance that global temperatures will rise by at least the 0.6°C warming of the twentieth century and
  • a 10 percent chance that temperatures will rise more than 4°C in the next century.

Other scientists are more certain of a larger rise, Tom Wigley of the National Center for Atmospheric Research in Colorado, USA and Sarah Raper of the University of East Anglia in England reported in 2001 that there was a 90% probability of average global temperatures rising 1.7 to 4.9 degrees over the next 100 years.

 Try the probability wheel at to see what the Civil Society Institute of America give as predicted outcomes of different size rises in sea temperature levels.

 Will it happen to me?

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Try it yourself

Read the following statements and assign them a level of probability between 1 and 100 where 100 = 100% certain:

1. It will rain later today.

2. You will get a ticket to the next Glastonbury Festival.

3. The sun will rise tomorrow.

4. Stem cell research will result in a cure for Parkinson’s disease.

5. A coin tossed fairly will land heads twice in a row.

Swap your answers with a partner’s and convert theirs into decimal fractions of one.

Need help?

Who was the most pessimistic?