In statistics, the intraclass correlation (or the intraclass correlation coefficient, abbreviated ICC) is a descriptive statistic that can be used when quantitative measurements are made on units that are organized into groups. It describes how strongly units in the same group resemble each other. While it is viewed as a type of correlation, unlike most other correlation measures it operates on data structured as groups, rather than data structured as paired observations.
The intraclass correlation is commonly used to quantify the degree to which individuals with a fixed degree of relatedness (e.g. full siblings) resemble each other in terms of a quantitative trait (see heritability). Another prominent application is the assessment of consistency or reproducibility of quantitative measurements made by different observers measuring the same quantity.
Read more about Intraclass Correlation: Early ICC Definition: Unbiased But Complex Formula, Modern ICC Definitions: Simpler Formula But Positive Bias, Relationship To Pearson's Correlation Coefficient, Use in Assessing Conformity Among Observers, Calculation in Software Packages