Reliability is a property of test scores from individuals who have been sampled from a well-defined population. Reliability indices, such as coefficient and related formulas for internal consistency reliability (KR-20, Hoyt's reliability), yield lower bound reliability estimates when (a) subjects have been sampled from a single population and when (b) test items are congeneric (i.e., when items are sampled from a single latent dimension). However, when samples are commingled--that is, when they are composed of scores that are drawn from multiple populations-- coefficient and related indices can be severely biased. In most cases the bias inflates; in other cases is attenuated. Equations are derived for elucidating this bias in two-group mixture distributions. (Contains 2 figures and 3 notes.)