What you'll learn
How to choose a representative sample and avoid bias.
Populations and samples
- Population: everyone/everything being studied. A census surveys the whole population.
- Sample: a smaller group selected to represent the population. A sampling frame is the list you sample from.
Sampling methods
- Simple random sampling: every member has an equal chance (e.g. random numbers).
- Systematic sampling: select every nth member from a list.
- Stratified sampling: split the population into groups (strata) and sample each in proportion.
- Quota and cluster sampling.
Bias
- A biased sample is unrepresentative. Reduce bias with random selection and a large enough sample.
Exam tips
- For stratified sampling, calculate each stratum's share: (group size ÷ total) × sample size.
- Justify why a method suits the situation.
Common mistakes
- Rounding stratified sample sizes wrongly.
- Confusing systematic with random sampling.