Educational assessment involves eliciting, transmitting, and receiving information concerning the level of proficiency of a learner in a specified domain. With that in mind, it is perhaps surprising that the literature seems to make very little use of the signal processing metaphor. The present article begins by making a general case for greater use of this metaphor, as a simple and intuitive thinking tool for helping to explain how educational assessment works. The main body of the article extends this argument by demonstrating the utility of the metaphor in helping to explain how educational assessment can go wrong. During the 1980s and 1990s, Samuel Messick extensively discussed two major ways in which educational assessment can go wrong via construct-irrelevant variance and construct underrepresentation, respectively. Despite their importance, these concepts have had only a limited impact on the literature. Part of the reason for this may be a lack of clarity and comprehensiveness in their articulation. The present article aims to articulate these concepts more clearly and comprehensively, within a framework provided by the signal processing metaphor, reconfiguring them as signal contamination and signal deficiency, respectively.