Test reliability is a concept central to classical test theory and it is commonly stated as a requirement that a test attain a certain level of reliability before it be considered of sufficient quality for practical use. This article discusses the role of reliability in item response theory, and in particular the role of reliability in contexts where matrix sampling designs are used and concern is with the estimation of population parameters rather than the measurement of individuals. The concept of a measurement design effect is introduced. This concept parallels the concept of sampling design effects, in that it describes the impact of measurement error at the individual level (described through a reliability index) on the accuracy with which population parameters are estimated.