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S1.3StatisticsCore

Non-Random Sampling

Sometimes a random sample is impractical, so non-random methods are used. The two you need are quota sampling and opportunity (convenience) sampling — quick and cheap, but more prone to bias.

20 min Data Collection
Edexcel AS Level Maths: Data Collection (Part 1) — Non-Random SamplingCurated by Kepler, not produced by us. Watch it before the notes below.
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What you'll be able to do

  • Describe quota sampling
  • Describe opportunity (convenience) sampling
  • Compare them with random methods
  • Recognise sources of bias
1

Quota sampling

The interviewer is told to select a fixed quota\textbf{quota} from each group (e.g. 20 men and 20 women) but chooses the actual individuals themselves. It does not need a sampling frame and is quick, but the interviewer’s choices can introduce bias.

2

Opportunity (convenience) sampling

You sample whoever is available\textbf{available} at the time — for example asking passers-by. It is the easiest and cheapest method but is the most likely to be unrepresentative\textbf{unrepresentative} and biased.

Tip — Opportunity sampling = “whoever is convenient”. Fast, but the least reliable for fair conclusions.

3

Random vs non-random

Random methods reduce bias but need a sampling frame and more effort. Non-random methods are faster and cheaper and work without a frame, at the cost of representativeness. The right choice depends on the situation.

Formula recap

quota: fixed number per group, chosen by interviewer\text{quota: fixed number per group, chosen by interviewer}
Quota sampling.
opportunity: whoever is available\text{opportunity: whoever is available}
Convenience sampling.
non-random⇒faster, but more bias\text{non-random} \Rightarrow \text{faster, but more bias}
Trade-off.

Common mistakes to avoid

Confusing quota sampling with stratified sampling.
Stratified is random within groups; quota lets the interviewer pick the individuals (non-random).
Assuming non-random samples are useless.
They are valid and practical — just more prone to bias and less representative.

Key takeaways

  • Quota: a fixed number from each group, chosen by the interviewer (no frame needed).
  • Opportunity: sample whoever is available — easiest but most biased.
  • Non-random methods trade representativeness for speed and cost.

Test yourself

Answer a few questions, then review anything you missed.

Next step

Keep practising

Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.

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Related lessons

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Put the methods into practice: see how A-Level maths duels work. An account is required to play; the overview and sample are public.