Sampling

Sampling is the process of selecting a subset of a population to study, on the reasoning that a well-chosen subset can support conclusions about the whole. The method chosen determines whether statistical generalisation to the population is defensible.

Probability methods give every unit a known, non-zero chance of selection and support inference: simple random, systematic, stratified (sampling within subgroups to guarantee representation) and cluster sampling. Non-probability methods do not: convenience, purposive, quota and snowball sampling are chosen for practicality or for reaching hard-to-access groups.

Most thesis research uses non-probability sampling for practical reasons, which is acceptable provided you name the method honestly and treat generalisability as a stated limitation rather than claiming it. Report the technique, the sampling frame, how participants were approached, the response rate and the final achieved sample.