# S-Cool Revision Summary

## S-Cool Revision Summary

### Simple probability

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 (unless you 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: p (not A) = 1 - p (A)

### Experimental probability

If you can 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.

## The 'and' rule

If you want one outcome or another outcome then you add their probabilities together.

For example:

For two events A and B, p (A or B) = p (A) + p (B)

The two events must be 'mutually exclusive'

### Tree diagrams  