Sampling is how you select the people or items you study from a larger population. The method you choose affects how far your findings can be generalised, so examiners pay close attention to it. This complete UK guide explains the difference between probability and non-probability sampling, the main techniques, and how to choose and justify your sample.
What Is Sampling?
Sampling is selecting a subset (the sample) from a larger population to study. Because studying everyone is rarely possible, the sample must be chosen carefully so your findings are meaningful and, where relevant, generalisable.
Probability vs Non-Probability
Probability sampling gives every member of the population a known chance of selection, allowing generalisation. Non-probability sampling does not, and is common in qualitative research where depth matters more than representativeness.
Probability Sampling Methods
✓ Simple random — everyone has equal chance.
✓ Systematic — every nth member.
✓ Stratified — by subgroups.
✓ Cluster — by groups.
Non-Probability Sampling Methods
✓ Convenience — who is available.
✓ Purposive — chosen for relevance.
✓ Snowball — participants recruit others.
✓ Quota — set numbers per group.
Choosing and Justifying Your Sample
Match the method to your approach and questions: probability methods suit quantitative, generalisable studies; non-probability suit qualitative, in-depth ones. Always justify your choice and acknowledge any limits to generalisability. See our limitations guide.
Common Mistakes and Tips
✓ Claiming generalisability from a convenience sample.
✓ No justification for the method.
✓ Sample too small to support claims.
✓ Confusing sampling types. Tip: match sampling to your approach and justify it clearly.
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Frequently Asked Questions
What is sampling in research?
Selecting a subset of a population to study because studying everyone is rarely possible.
What is the difference between probability and non-probability sampling?
Probability sampling gives a known chance of selection and allows generalisation; non-probability does not.
What are probability sampling methods?
Simple random, systematic, stratified and cluster sampling.
What are non-probability sampling methods?
Convenience, purposive, snowball and quota sampling.
Which sampling suits qualitative research?
Usually non-probability methods such as purposive sampling.
Which sampling suits quantitative research?
Usually probability methods that support generalisation.
How do I justify my sample?
Match it to your approach and questions and acknowledge any limits.
What is a common sampling mistake?
Claiming generalisability from a non-representative convenience sample.
Related Study Guides
How to Write a Methodology • Qualitative vs Quantitative Research • How to Write a Limitations Section • How to Write a Dissertation
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