Rho

A statistic that quantifies the extent to which population units within clusters are similar to one another [Page 765](i.e. the degree of homogeneity within clusters) is called the intraclass correlation coefficient (ICC) and is often denoted by the Greek letter rho (ρ).

When population units are grouped or clustered into larger units, which are themselves easier to identify and sample (i.e. children grouped into classrooms or elderly citizens grouped into nursing homes), one-or two-stage cluster sampling becomes an appealing, cost-effective, and practical choice for a sampling strategy. These benefits are often counterbalanced by the usual expected loss in efficiency and precision in estimates derived from cluster samples that is, in large part, due to the fact that units within clusters tend to be more similar to ...