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:

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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:

Tree Diagrams