Everyone vs the part you measure: The population is every individual or item you want to know about.
A sample is the smaller part you actually collect data from.
The exam gives you a context and asks you to identify each.
IB-style question — name the population and sample
A vet wants the average mass of every golden retriever registered in a city.
She weighs 40 of them.
Identify the population and the sample.
Step by step
- Population = everyone you want to know about.
- Sample = the part actually measured.
Final answer
Population: all golden retrievers registered in the city. Sample: the 40 she weighs.
Ask: who is the question really about?: The population is the whole group the conclusion will be about — not just the ones measured.
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A census measures everyone; a sample measures part: A census collects data from the whole population; a sample uses only part of it.
A common exam question asks why a sample is used — usually cost, time, or because the test destroys the item.
IB-style question — give a reason
A factory makes 50 000 light bulbs a day and wants to know their average lifetime.
Explain why it tests a sample rather than every bulb.
Step by step
- Measuring a bulb's lifetime means running it until it fails — that destroys it.
- A census would destroy the whole day's stock.
Final answer
Because the test destroys each bulb — a census would ruin all the stock, so only a sample is tested.
Three standard reasons: Sampling is cheaper, faster, and sometimes the only option (destructive testing).
Quote one in a 'give a reason' part.
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A reliable sample represents the population: A sample is reliable when it represents the whole population.
A biased sample over- or under-represents part of it, so its results don't generalise.
Fairly chosen, larger samples are more reliable.
IB-style question — spot the problem
A school surveys opinions on its sports facilities by asking only members of the football team.
Explain why this sample is unlikely to be reliable.
Step by step
- The football team already favours sport.
- So the sample is biased toward one type of student.
Final answer
The football team is not representative — it's biased toward sporty students, so the results won't reflect the whole school.
Bigger isn't enough on its own: A huge but unfair sample is still biased.
Reliability needs a fair selection method (next micro) as well as a reasonable size.