Pure chance can miss a group
A school has 12 pupils in Year 9 and 8 in Year 10 in a small survey list. A random pick of 5 might give all five from Year 9.
Other methods make sure every group is heard, or make the pick quick and easy.
Key words
| Word | What it means |
|---|---|
| Stratified sampling | sample each group in proportion to its size |
| Stratum | one group in the population, such as a year group |
| Cluster sampling | pick whole groups at random and use everyone in them |
| Systematic sampling | pick every few items from a list, from a start point |
| Sampling interval | the gap between picked items |
| Representative | reflecting the whole population |
| Bias | a built-in unfairness that skews a result |
The cells students fill in are left blank in this preview.
By the end of this lesson
Note
I can choose and apply stratified, cluster or systematic sampling.
Starter
Have a go
Starter
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.
Stratified sampling hears every group
Split the population into groups called strata. Take a random sample from each group, in proportion to its size.
Use it when groups differ and each must be represented, such as year groups.
Worked example: 10 from 100 pupils
| Group | Pupils in group | Pupils to sample |
|---|---|---|
| Year 9 | 60 | 6 |
| Year 10 | 40 | 4 |
| Total | 100 | 10 |
The cells students fill in are left blank in this preview.
Try it: two from each year group
select year_group, pupil_id
from survey
qualify row_number() over (partition by year_group order by hash(pupil_id)) <= 2
order by year_group, pupil_id;
-- Try it: change 2 to 3.
Check
Have a go
Check
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.