Sampling methods
Random, systematic, stratified, opportunity, quota. Bias is the enemy.
Y12 Stats. Define population, sampling frame, sample. Each method has trade-offs in bias and convenience. Random and stratified are unbiased; opportunity and quota can be biased.
Worked examples
Random sample: every individual equally likely.
Stratified: split population into groups, sample proportionally from each.
Systematic: every kth from a list. Convenient but not random.
Frequently asked questions
When to use stratified?
When the population has distinct subgroups (men/women, age bands) and you want representation.
Why is opportunity sampling biased?
It overrepresents those most easily reached.