This document describes different sampling methods used to select a representative sample from a larger population. It defines sampling as selecting a smaller group to represent the whole population since surveying every individual is not feasible. Two common sampling methods are described: systematic sampling, which selects every nth individual; and stratified sampling, which divides the population into groups and samples from each group to ensure all subgroups are represented. The key is to use a sampling method that avoids bias and results in a sample that accurately reflects the characteristics of the overall population.

1. sampling
sec 2 geography

2. objective
describe how systematic sampling is done.
describe how stratified sampling is done.

3. sampling
= selecting a smaller group (sample)
to represent the whole population
Because it is too expensive and not humanly possible to
ask every single person!

4. eg:
I want to find out how the whole of Sembawang thinks
about Sembawang Secondary School.
Not possible isn’t it?
So we have to select a sample that will represent the
whole Sembawang population.

5. sampling
= selecting a smaller group (sample)
to represent the whole population
sampling method
= how we select a smaller group (sample)
so that it will be able to represent the whole population

6. eg: this is the population
population: 100
25 25 25 25
I can’t survey everyone.

7. eg: this is the population
population: 100
25 25 25 25
I can’t survey everyone.
So I choose to survey 25
people instead.

8. If we don’t do sampling properly, the sample
wouldn’t represent the population
population: 100
25 25 25 25

9. If we don’t do sampling properly, the sample
wouldn’t represent the population
population: 100
25 25 25 25
eg: I sample the first 25 people I see.
sample: 25
15 10 0 0
3/5 2/5 0/5 0/5
sample has a different
proportion as population
= NOT representative
Sample represents
more than the rest
= bias

10. If we don’t do sampling properly, the sample
wouldn’t represent the population
population: 100
25 25 25 25
eg: I sample the last 25 people I see.
sample: 25
0 0 15 10
0/5 0/5 3/5 2/5
sample has a different
proportion as population
= NOT representative
Sample represents
more than the rest
= bias

11. sampling method
= how we select a smaller group (sample)
so that it will be able to represent the whole population
Method 1: Systematic
Every X number of person, we will ask them
Method 2: Stratified
Ensure that all groups are being asked.
To make sure our sample is representative of the
population + not bias, we use certain sampling methods
to avoid bias.

12. eg: this is the population
population: 100
25 25 25 25

13. I do systematic sampling
Eg: survey every 5th person
population: 100
25 25 25 25
1/4 1/4 1/4 1/4
sample: 25
5 5 5 5
1/4 1/4 1/4 1/4
sample has a similar
proportion as population
= representative

14. But what if the population is
arranged like this?
population: 100
25 25 25 25
1/4 1/4 1/4 1/4

15. if I do systematic sampling
population: 100
25 25 25 25
1/4 1/4 1/4 1/4
25 0 0 0
4/4 0/4 0/4 0/4
sample does not have a
similar proportion as
population
= NOT representative
sample: 25

16. which is why we can also do stratified sampling
= ensure every group is surveyed
population: 100
25 25 25 25
1/4 1/4 1/4 1/4
sample: 25
5 5 5 5
1/4 1/4 1/4 1/4
sample have a similar
proportion as population
= representative

17. sampling method
1) To avoid bias
2) To make sure that the sample is
representative of the population
which
we choose depends on how the population is distributed