GCSE Data Handling Coursework
Stratified Sampling
Simple random sampling means that each record in the population
has the same chance of being selected. Methods to select a
random sample include using a spreadsheet to generate random
numbers, pulling numbers out of a hat, throwing a coin or die etc
One problem is that you might get a sample that does not really
reflect the population eg not enough girls etc
Stratified sampling ensures that the ratio of important categories
in the sample remains similar to that in the population.
Population
Sample
1
GCSE Data Handling Coursework
Stratified Sampling (Part1)
Eg The Exams Board would like a sample from four schools of maths
coursework. They would like a sample of 50 pupils. Look at the
table below. How many pupils should each school provide?
School
Pupils
A
315
B
123
C
95
D
217
Answer Total Pupils =315+123+95+217=750
A
315 x 50 = 21 pupils
750
B
123 x 50 = 8.2 ie 8 pupils
750
C
95 x 50 = 6.33 ie 6 pupils
750
D
217 x 50 = 14.5 ie 15 pupils
750
For a random stratified sample 21 pupils should be picked at
random from school A, 8 from B etc
2
GCSE Data Handling Coursework
Stratified Sampling (Part2)
Eg The Exams Board would now like a 50 strong sample from 4 schools
that’s representative of the male/female breakdown. How many
males/females should each school provide? (Use the school sample
from Part 1)
Answer Total Pupils = 750
School
A
F
M
150 165
Pupils
A 21
150 x 21 = 10 females 11 males
315
B8
61 x 8 = 4 females 4 males
123
C6
60 x 6 = 4 females 2 males
95
D 15
122 x 15 = 8 females 7 males
217
315
B
61
62
123
C
60
35
95
D
122
95
217
For a random stratified sample 10 female & 11 male pupils should
be picked at random from school A, etc
3