The primary objective of a human abuse potential (HAP) study is to provide information on the relative abuse potential of a test drug (generally a central nervous system (CNS)-active new molecule entity (NME)) in humans. Data from HAP studies are important in developing abuse-related drug product labeling and in determining whether the drug product will be scheduled under the Controlled Substances Act (CSA). In HAP studies, visual analog scales (VAS, 0 - 100) are used for primary measure(s) and some secondary measures. For example, Drug Liking, and Take Drug Again are on a bipolar VAS scale (0=maximum negative response, 50=neutral response, 100=maximum positive response); High, and Good Effects are on a unipolar VAS scale (0=maximum negative response, 100=maximum positive response). Even though training for study subjects on these different scales is conducted before the study, study subjects can still be confused by use of these different scales during the study. In this presentation, I will use examples to show how serious this problem can be, and how to diagnose the problem as well as provide methods to eliminate the problem in future studies.