In contrast to Pearson's correlation in which the relationship between two variables (measures such as the height and body weight of a person) is concerned, intraclass correlation (ICC) is defined as the correlation of one variable (measure) between two or more members within groups. Applied to the context of brain imaging, ICC can be used as an indicator of scanning reliability or consistency across sessions/days/sites, for example, and, defined as the proportion of variability across subjects relative to the total variability in the data.

Here we provide a program, 3dICC, to calculate the ICC on brain volume data instead of ROI data typically seen in the literature. If there is one random factor (e.g., session) other than subject, the ICC is typically calculated in a two-way random-effects ANOVA model as shown in the classical notation of ICC(2,1) [Shrout and Fleiss (1979), Psychological Bulletin, Vol. 86, No.2, 420-428]. In the same token, here I've extended the definition of ICC to two random factors in addition to subject in a three-way random-effects ANOVA.

More details can be found at

[afni.nimh.nih.gov]
http://afni.nimh.nih.gov/sscc/gangc/ICC

Gang