Key Idea: A sample is a smaller group taken from a population. A good sample mirrors the population. You choose a sampling method and judge whether the data are reliable and valid.
| Method | How |
|---|---|
| Simple random | Everyone has an equal chance |
| Systematic | Every kᵗʰ person |
| Stratified | Groups in proportion |
| Quota | Interviewer fills targets |
| Convenience | Whoever is easiest; fastest but most biased |
Key ideas
- Random methods (simple random, systematic, stratified) reduce bias. Quota and convenience are quicker but risk bias.
- Stratified count = (group ÷ total) × sample size.
- Reliable = same result on repeat (small random error).
- Valid = measures the right thing (no systematic bias).
Reliable means consistent; valid means aimed at the right target.
250 under 30, 150 aged 30–50, 100 over 50; a sample of 40. How many from 30–50? 150/500 × 40 = 12.
Why stratify by age here? Each age group appears in proportion, so even the smallest group is not missed.
A scale always reads 2 kg too heavy. Reliable, valid, both or neither? Reliable but not valid: consistent, but biased.
A student asks only friends in the canteen. Which method? Convenience.