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.
What you'll be able to do
- Describe quota sampling
- Describe opportunity (convenience) sampling
- Compare them with random methods
- Recognise sources of bias
Quota sampling
The interviewer is told to select a fixed 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.
Opportunity (convenience) sampling
You sample whoever is at the time — for example asking passers-by. It is the easiest and cheapest method but is the most likely to be and biased.
Tip — Opportunity sampling = “whoever is convenient”. Fast, but the least reliable for fair conclusions.
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
Common mistakes to avoid
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
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