To gain a sense of the persuasive efficacy of a message prior to implementation of a campaign, researchers often gather judgments of perceived effectiveness (PE). At present, they do so without much knowledge of the conceptual meaning or empirical properties of PE. In the spirit of construct explication, we report a study intended to address a series of questions about PE. Using student (N = 155) and community samples (N = 100), we found the following: (a) PE is a two-dimensional judgment involving global evaluations of message impact and specific judgments of message attributes, but it may be reducible to a single second-order factor, (b) most individuals reported using more than one referent (i.e., person or group) when making PE judgments, but the choice of referents varies by message and judge, and (c) judgments of PE are biased upward as a function of the number of referents chosen. Suggestions are offered for enhancing the validity of PE judgments in formative campaign research.