Conventional methods for evaluating the utility of subscores rely on reliability and correlation coefficients. However, correlations can overlook a notable source of variability: variation in subtest means/difficulties. Brennan introduced a reliability index for score profiles based on multivariate generalizability theory, designated as [Formula: see text], which is sensitive to variation in subtest difficulty. However, there has been little, if any, research evaluating the properties of this index. A series of simulation experiments, as well as analyses of real data, were conducted to investigate [Formula: see text] under various conditions of subtest reliability, subtest correlations, and variability in subtest means. Three pilot studies evaluated [Formula: see text] in the context of a single group of examinees. Results of the pilots indicated that [Formula: see text] indices were typically low; across the 108 experimental conditions, [Formula: see text] ranged from .23 to .86, with an overall mean of 0.63. The findings were consistent with previous research, indicating that subscores often do not have interpretive value. Importantly, there were many conditions for which the correlation-based method known as proportion reduction in mean-square error (PRMSE; Haberman, 2006) indicated that subscores were worth reporting, but for which values of [Formula: see text] fell into the .50s, .60s, and .70s. The main study investigated [Formula: see text] within the context of score profiles for examinee subgroups. Again, not only [Formula: see text] indices were generally low, but it was also found that [Formula: see text] can be sensitive to subgroup differences when PRMSE is not. Analyses of real data and subsequent discussion address how [Formula: see text] can supplement PRMSE for characterizing the quality of subscores.