The double-entry intraclass correlation as an index of profile similarity: meaning, limitations, and alternatives

As applied to many psychological phenomena, analysis of profile similarity has an intuitive appeal that masks complex statistical issues. Psychologists have long debated the methods of indexing similarity between 2 psychological profiles, but the double-entry intraclass correlation may be emerging as a preferred approach. Unfortunately, the double-entry intraclass correlation has not been articulated clearly in terms of fundamental facets of profiles--elevation, scatter, and shape--which prevents full understanding of its meaning. In this article, I (a) articulate these effects, (b) discuss potential limitations and confusions arising from these effects, (c) present a failure to replicate previous empirical findings regarding the double-entry intraclass correlation, and (d) present alternative recommendations for analysis of profile similarity. The conceptual, mathematical, and empirical points may enhance the insights emerging from analyses of profiles and profile similarity.