I read the explanation of reasons for finding low ICC values in chapter 26 of the book, Foundations of Clinical Research application to practice.
One of the reason is variance. The authors said "the variability among subjects' socres must be large to demonstrate reliability. A lack of variability can occur when samples are homogeneous, when raters are all very lenient or strict in their scoring, or when the rating system falls within a restricted range........If subjects' socres are homogeneous, this source of variance will not be significant....Therefore, the researchers have to try to obtain heterogeneous samples whenever possible."
My question is that I don't really understand the meanings of homogeneous and heterogeneous mentioned here. Did the authors suggest that we have to recruit the clients with different diagnoses in order to obtain heterogeneous samples??