Source identification refers to memory for the origin of information. A consistent nomenclature is introduced for empirical measures of source identification which are then mathematically analyzed and evaluated. The ability of the measures to assess source identification independently of identification of an item as old or new depends on assumptions made about how inconsistencies between the item and source components of a source-monitoring task may be resolved. In most circumstances, the empirical measure that is used most often when source identification is measured by collapsing across pairs of sources (sometimes called "the identification-of-origin score") confounds item identification with source identification. Alternative empirical measures are identified that do not confound item and source identification in specified circumstances. None of the empirical measures examined provides a valid measure of source identification in all circumstances.