Cluster sampling
Clusters may be branches, stores or geographic areas. Depending on the design, all units in selected clusters or a further sample within them are examined. Clustering can reduce travel or collection cost but similar units within a cluster can reduce precision; analyse the design rather than treating every observation as independent.
Worked example
A company randomly selects five branches and audits invoices within those branches. The branch is the cluster; conclusions should account for the selection design.
Common mistake
Confusing cluster sampling with stratified sampling: stratification samples from each selected stratum, whereas cluster sampling selects groups.
Where it appears in your exams
| Qualification | Exam | Relationship | Period |
|---|---|---|---|
| ACCA | MA | Direct | September 2026–June 2027 |
| FIA | FMA | Direct | September 2026–June 2027 |