What it shows
In purposive sampling, also called judgment sampling, the researcher selects the units judged most useful for the study. In the diagram, 14 of the 120 units have a characteristic the study needs, and 8 of those are selected. Units without it are not considered.
For the illustration, seeded random draws set which units carry the characteristic and which 8 are chosen. In a real study the choice rests on the researcher’s stated reasoning, normally set out in the methodology chapter.
How to read it
A dark outline marks a unit that has the characteristic.
A dark outline with a filled center marks a unit that has it and was selected.
Plain open circles are the rest of the population.
The numbers behind it
- Population of 120 units.
- 14 units have the characteristic; 8 of them are selected.
Ways to use it in teaching
Each student writes one sentence justifying their selection criteria; compare the sentences as a group.
Pairs well with snowball sampling in a qualitative methods session, since the two are often combined.
Discuss what a purposive sample is for: depth on a specific question, not generalization to a whole population.
For when each method suits a study, and how to justify the choice in a methodology chapter, see our guide to sampling methods.
