The category-partition method and the classification-tree method help construct test cases from specifications. In both methods, an early step is to identify a set of categories (or classifications) and choices (or classes). This is often performed in an ad hoc manner due to the absence of systematic techniques. In this paper, we report and discuss three empirical studies to investigate the common mistakes made by software testers in such an ad hoc approach. The empirical studies serve three purposes: (a) to make the knowledge of common mistakes known to other testers so that they can avoid repeating the same mistakes, (b) to facilitate researchers and practitioners develop systematic identification techniques, and (c) to provide a means of measuring the effectiveness of newly developed identification techniques. Based on the results of our studies, we also formulate a checklist to help testers detect such mistakes.