Debates continue in Japan over replacing the National Center Test for University Admissions (hereafter, the Center Test) in the English sections of university entrance exams. Although the use of external tests—such as the EIKEN Test and TOEFL iBT—has been considered, retrospective evaluations of the Center Test’s washback effects (i.e., the influence of testing on teaching and learning) remain scarce. In discussions surrounding the introduction of the new Common Test in 2025 or the adoption of external tests, it is essential to first examine the longstanding Center Test. Such analysis can help uncover underlying mechanisms of washback in the current context of English education in Japan.

Despite the Center Test’s widespread influence, the role of shadow education—particularly private preparatory schools (yobiko) and cram schools (juku)—in mediating its effects has received limited scholarly attention. Notably, these institutions not only provide instruction but also design and administer mock tests. Students use these practice tests, which closely mirror the format and content of the official exam, to assess their performance and make informed decisions about university applications. However, the extent to which yobiko-developed mock tests differ from the official Center Test has not been systematically investigated. These differences may exert indirect effects on learners through washback, thus warranting further examination.

This study investigates the washback effects of the Center Test’s English section, focusing on its influence on teaching and learning. Specifically, it compares official test items from the 2018 Center Test with three sets of mock tests produced by major yobiko institutions to explore the potential indirect impact of shadow education.

The findings are as follows: In the text-based analysis, readability indices revealed that while the mock tests were more difficult than the official test in Reading Section Part 6, overall difficulty levels were comparable. Vocabulary indices indicated that the official test was generally easier than the mock tests. In the coding analysis, the reading skills assessed were largely consistent between the official and mock tests. By focusing on one dimension of the Center Test’s washback effect, this study sheds light on how large-scale standardized tests in Japan shape teaching and learning.