When developing a tool to measure any physical or psychological trait, the degree to which the tool actually measures what it is supposed to with sufficient consistency is of the utmost concern. From a measurement perspective, the “actually measures what it is supposed to” part of the previous statement refers to a property of instruments called validity, whereas the “with sufficient consistency” part refers to a property called reliability. Both of these properties are equally important from a test development standpoint, and establishing reliability is a prerequisite for assessing validity. This entry discusses reliability within the context of classical test theory (CTT). First, reliability is defined conceptually, followed by a discussion of the importance of ensuring that psychological instruments contain items that are equally reliable ...