Washback refers to the influence of a test on teaching and learning, arising both directly and through mediating factors. The washback of the Common Test for University Admissions (the Common Test) can arise not only from its direct impact but also from mediating factors. One such mediating factor in relation to the English section of the Common Test is the extent to which commercially available mock examinations accurately reflect the actual test. Against this background, the present study analyzes the English reading sections of mock examinations produced by major preparatory schools (yobiko), aiming to clarify how these mock tests mediate the washback effect.

Yobiko generally produce two types of mock examinations. The first type is the “workbook,” which compiles past mock examinations and is typically released early in the test preparation period (e.g., in the spring), containing multiple test forms. The second type is the “final-prep pack,” a comprehensive package of mock tests designed to simulate the actual Common Test closely. Because these packs are sold and used immediately before the examination and often contain only a single test set for each subject, they are likely to have greater washback intensity than workbooks.

To examine how closely these final-prep packs reflect the actual test, this study compared the vocabulary level and readability of the Common Test English Reading test with those of final-prep packs published by three major yobiko. The passages from each test were converted into plain text to compute vocabulary and readability indices. The results showed no statistically significant difference in vocabulary levels between the actual test and the commercially available packs. However, readability indices indicated that the final-prep packs were significantly more difficult than the actual test. This gap suggests a potential source of negative washback, as it may lead examinees to perceive the Common Test as more difficult than it actually is, potentially affecting test-takers’ test preparation.