Sampling Methods and Stratification
You cannot always ask everyone. Learn to pick a fair sample: random, systematic and stratified methods, plus the proportional maths examiners love.
Revise this, the fun way
Play it interactively, earn XP and build a streak, free.
Start revising freeWhat you'll cover
The whole or a part? 👥
The whole group you want to study is the **population**. Collecting data from **every** member is a **census**. Usually that is too **slow**, too **expensive**, or impractical. So you study a **sample**: a smaller subset chosen to stand in for the whole. The risk is **bias**: if the sample is not **representative**, its results will not reflect the population fairly.
The key word
What is a census?
- Collecting data from every member of the population
- Studying a small subset of the population
- Picking every tenth person on a list
- Choosing whoever is nearest to hand
Giving a fair chance 🎲
Two methods give everyone a fair chance:
Match each method to its description
- Simple random
- Systematic
- Stratified
- Convenience
- Every member has an equal chance of selection
- Pick every nth member from an ordered list
- Sample each group in proportion to its size
- Use whoever is easiest to reach
Name the method
A researcher lists all 500 members and selects every 25th person. Which method is this?
- Systematic sampling
- Simple random sampling
- Stratified sampling
- Quota sampling
Stratified sampling 📊
Split the population into **strata**, groups that share a feature such as year groups, then sample **each stratum in proportion** to its size. To find how many to take from a stratum: **(stratum size / population size) x sample size.**
Stratified calculation
An interactive activity.
One more
An interactive activity.
True or false
Select the TWO true statements about sampling.
- Stratified sampling keeps each group in proportion
- A biased sample is not representative of the population
- A census always uses fewer resources than a sample
- Convenience sampling is the least likely to be biased
Quick but risky 🎯
Some methods trade fairness for speed: - **Convenience** (opportunity) sampling uses whoever is easiest to reach. It is quick but often **biased**. - **Quota** sampling fills fixed numbers for each group, but the members are chosen **non-randomly** by the researcher.
Pick the method
An interactive activity.
Sampling frame
The whole group is the _____; a subset used to represent it is a _____. Simple random sampling gives everyone an equal chance, while _____ sampling takes each group in proportion. A sample that is not representative is _____.
The big picture
Why is stratified sampling often preferred?
- It keeps each group in proportion, so the sample is representative
- It is always the fastest possible method
- It never needs any calculation
- It only ever uses the nearest people