The argument-based validation research reported in this chapter was conducted from the perspective of an outside evaluator with concerns about the consistency of scores on the Telephone Standard Speaking Test (TSST), a telephone-based test of second language (L2) English speaking proficiency used to assess improvement in speaking proficiency over time. The test use requires that the warrant for generalization be plausible. It states that observed scores are estimates of expected scores, which are consistent across test tasks, forms, occasions, and raters. To guide the investigation a rebuttal, that observed scores fail to estimate expected scores due to error introduced in the testing process, was formulated. The research investigated two of its assumptions. Data of the TSST scores from 55 undergraduates at two Japanese universities collected twice within a month indicated that test forms had the same means and the same SDs, and that the two scores of each participant were highly correlated. One-third of scores for the same individual differed by one score level. Thus, the results found partial support for one of the assumptions underlying the rebuttal. This chapter concludes by highlighting the important role of rebuttals for including threats of concern to test users in an interpretation/use argument.