Intraclass Correlation Coefficient

The value of the population intraclass correlation coefficient is a measure of the homogeneity of observations within the classes of a random factor relative to the variability of such observations between classes. It will be zero only when the estimated effect of the random factor is zero and will reach unity only when the estimated effect of error is zero, given that the total variation of the observations is greater than zero (see Hays, 1988, p. 485).

Note that the population intraclass correlation can be estimated using the variance component estimation methods available in the Variance Components and Mixed-Model ANOVA/ANCOVA method of analysis.