AS 912674 creditsExternal

2.12 Probability Methods

Note 1: Basic Probability

• Probability is how likely it is that an event will occur

• It is measured on a scale of 0 (impossible) to 1 (certain)

• It can be written as a fraction or a decimal

Note 2: Experimental Probability

The results of experiments allow us to determine the relative probabilities from patterns in the past

P(Event) = number of favourable outcomestotal number of outcomes

Example: There are 200 students at the local high school, 94 of these students are in the junior school.

P(student is in the junior school) = 94/200

The probability that an event will not occur = 1 – P(event)

P(student not in the junior school) = 1 – 47/100 = 53/100

Note 3: Two-way Tables

• A two-way table has rows which represent one variable and columns that represent another

• Each cell represents the frequency (or proportion) belonging to that particular row and column

Example:

The following table shows the favourite subject of a group of year 9 students:

English Maths Science TOTAL

Boys 12 24 11 47

Girls 9 30 14 53

TOTAL 21 54 25 100

A student is selected at random. Find

P(a boy whose favourite subject is Maths) =

P(Girl) =

24/100

53/100

Page 168Exercise A