GCSE: AQA Unit 1 [calculator allowed]

The Rationale behind Sampling

When doing a survey it is often impossible to ASK EVERYBODY what their opinion might be.

For this reason, we normally TAKE A SAMPLE.

However, IT IS IMPORTANT that a sample is REPRESENTATIVE of the POPULATION

STRATIFIED SAMPLING

When a population is divided up into groups, taking a STRATIFIED SAMPLE ensures that each GROUP is asked in the CORRECT PROPORTION

EXAMPLE – how to find a stratified sample

A teacher wants a sample of 50 people from a school. The school is divided into three houses. How many people should be asked from each house?

 Red House Green House Blue House 327 246 401

a)    The total number of pupils is b)    To find the number of students from each house to be asked, we multiply each house by House Red House Green House Blue House Sample size      Round the numbers   You will wonder why we rounded the last column DOWN to 20.   We only wanted a sample of 50, and rounding it up to 21 would have given a sample of 51.   I chose to round the third column down because it was the lowest of the three decimals

Key points:   