# 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

**Here's a diagram to help you understand them:**