Question 1: Why do the authors refer to the intra-class correlation coefficient as a "relative" reliability index?
The ICC is the ratio of the variance of the thing being measured to the variance of the measurement of the thing. In other words, it is the variance of the true values divided by the variance of the measured values, which is the variance of the true values plus the measurement error variance.
Hence as an index of measurement error, ICC is relative to the variability in the population being measured.
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