You are here


Probability is the 'chance' of something happening.

It is almost certain to come up in your exam! (Nothing is ever completely certain!)

Key points:

1. We call 'something happening' an event. For example, getting a 6 with the roll of a dice.

2. Probability should always be written as a fraction, decimal or percentage never '1 in 10' or '3 chances out of 5'.

3. The probability of something happening must be between 0 and 1 (unlessyou are using percentage - 0 to 100).

4. The sum of the probabilities of every possible outcome is 1.

5. The probability of something not happening is 1 minus the probability of it happening.

If we call a particular event 'A' then the probability of A happening is:

Copyright S-cool

For example:

p (getting an even number when you roll a dice) =

Copyright S-cool

And p (not A) = 1 - p (A)

For example:

If p (rolling a 6 on a dice)

Copyright S-cool

Then p (not rolling a 6)

Copyright S-cool

In practice, a coin will not always land 'Heads' exactly half the times you throw it (although theoretically it should!).

If you do something a certain number of times and record the results you can write down estimates for the probabilities of each outcome.

These estimates are also known as Relative Frequencies.

Here's an example:

A ball was drawn out of a bag containing coloured balls 200 times. Each time the ball was replaced. The results are shown below...

Work out all the probabilities and write them in the boxes:

Copyright S-cool

You don't need to bother cancelling the fraction down to its simplest terms in this type of question. You will still get full marks.

Why take the risk?

S-cool exclusive!!