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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: