When data are clustered (i.e., unit observations nested within clusters), researchers often wish to know how units within clusters are correlated. For example, it may be useful for rater reliability studies to know how the ratings of judges (units) are correlated within targets (clusters). Another example from survey research is how individuals’ responses (units) are correlated within neighborhoods (clusters). These questions can be answered with intraclass correlation (ICC or ρ) coefficients, which are ratios of the variance associated with the cluster variable in question to the total variance. These ratios can be interpreted as a correlation coefficient (i.e., how units are correlated within clusters).

In addition to answering substantive questions, ICCs are important parameters in a priori power calculations for experiments or precision estimates for surveys ...

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