Sampling Methods and Stratification
You cannot survey everyone, so you take a sample. Learn the main sampling methods, why some are fairer than others, and how stratified sampling shares a sample out in proportion.
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You cannot ask everyone
To study a large group, you cannot survey every single person. Instead you take a sample: a smaller group chosen to represent the whole population. The way you choose that sample matters. This module covers the main sampling methods and how stratified sampling shares a sample out fairly.
Four ways to sample
Four sampling methods. Learn what each one does.
Random or systematic?
Two common methods work in different ways.
Match the method
- Random sampling
- Systematic sampling
- Stratified sampling
- Opportunity sampling
- Every member has an equal chance
- Choose every nth member from a list
- Sample each group in proportion
- Choose whoever is available
Every nth person
Which sampling method chooses every nth person from a list?
- Systematic sampling
- Random sampling
- Stratified sampling
- Opportunity sampling
Stratification
Stratification is a pre-sampling step: you divide the population into strata, meaning groups such as year groups. It is NOT itself a sampling method. In stratified sampling you then sample each stratum in proportion to its size, using the sampling fraction, which is the proportion of the population you sample.
Match the word
- Population
- Stratum
- Sampling fraction
- Stratification
- The whole group you are studying
- One group within the population
- The proportion of the population you sample
- Dividing the population into groups before sampling
Reducing bias
Which TWO sampling methods give members a fair chance and reduce bias? Choose all that apply.
- Random sampling
- Stratified sampling
- Opportunity sampling
- Choosing only your friends
Work out the sample
A stratum has 150 people and the sampling fraction is 10 percent. How many people are sampled from this stratum?
Stratified sampling steps
Put the steps of stratified sampling into a sensible order.
- Divide the population into strata
- Find the size of each stratum
- Apply the sampling fraction to each
- Sample randomly within each stratum
Name the sampling method
A smaller group chosen to represent the whole is a _____. Choosing every nth person from a list is _____ sampling. Giving every member an equal chance is _____ sampling. Sampling each group in proportion is _____ sampling. Dividing the population into groups before sampling is called _____.
Spot the sampling methods
Tap the TWO that are sampling methods, not a pre-step or a group.
- Random sampling
- Systematic sampling
- Stratification
- The population
Name the method
For each procedure, choose the sampling method it describes.
- You choose every tenth name on the register. Which method?
- You split students by year group and sample each in proportion to its size. Which method?
- You ask whoever happens to walk past the school gate. Which method?
Explain the method
Explain stratified sampling and why it can be better than opportunity sampling.
- Explain how stratified sampling works
- Explain the problem with opportunity sampling
- Use the word proportion