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ICC - the Intra-class correlation coefficient

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Intra-class correlation coefficient
A measure of how similar members of groups are compared to how similar they are to others outside their groups. How much birds of a feather flock together.

Think of a study looking at students from two schools.
Scenario 1: one all boys school and one all girls school. If you know the school, you know the sex of the child. The ICC is 1.
Scenario 2: Both schools have exactly 40% boys, 60% girls. Knowing the school tells you nothing about the sex. The ICC is 0.

These extreme examples obscure something: The difference between the groups only partially drives the ICC. The variability within the group matters as well. Two other pairs of schools with 10% difference in rates (e.g. 40% and 50% girls, 10% and 20% girls) will have different ICC's.