icc

icc(values, subject_col, value_col)

ICC(2,1): two-way random effects, absolute agreement, single measure.

The variant matters and papers rarely say which they used. ICC(2,1) is the one that answers “would a repeat measurement of this person give the same number”, which is the question a clock’s technical reliability is about. ICC(3,1) assumes the raters are the only ones of interest and reports a higher number for the same data.