What it shows
In cluster sampling the population is grouped into clusters and a random sample of whole clusters is drawn. Here 120 households sit in 20 city blocks of 6; 2 blocks are drawn at random and every household in them is included, 12 in all.
Only the blocks are numbered. The sampling frame is a list of clusters, not of households, which makes the method usable when no full list of the population exists.
How to read it
Each outlined group of six circles is one city block, the cluster.
The two blocks with dark outlines were drawn at random from the list of 20.
Every household in a selected block is filled; no household in any other block can enter the sample.
The numbers behind it
- Population of 120 households in 20 city blocks of 6.
- Simple random sample of 2 blocks (blocks 6 and 17), so 2 × 6 = 12 households.
- Every household has the same chance of selection, 2 in 20 or 1 in 10.
- The two blocks were drawn with a seeded random number generator (mulberry32, seed 20261220), so the draw can be repeated.
Ways to use it in teaching
Contrast it with stratified sampling: there a sample is drawn from every group, here only the selected groups are sampled.
Ask why households in the same block may answer alike, and what that does to precision compared with a simple random sample of 12 households.
Use the multistage diagram next, which samples again inside each selected cluster instead of taking all of it.
For when each method suits a study, and how to justify the choice in a methodology chapter, see our guide to sampling methods.
Sources
- Ahmed, S. (2009). Cluster sampling [Lecture 5, Methods in sample surveys, 140.640]. Johns Hopkins Bloomberg School of Public Health OpenCourseWare.
- Cluster sampling. (n.d.). In Wikipedia. Retrieved October 19, 2026, from https://en.wikipedia.org/wiki/Cluster_sampling
