Beginning in September 2020, students will have the option to retake one or more sections of the ACT® test (referred to as section retesting, modular testing, or single-subject retesting), instead of needing to take the entire battery again. Section retests will only be available to students who have previously completed the full battery and only available to students retesting online. The section retest option is being made available to students because research conducted to date indicates that ACT scores combined across multiple administrations are valid; this includes results from the current study which suggest that students' performance when retesting in a single ACT subject area tends to be consistent with expected performance estimated from standard retesting with the full battery. The focus of the current study was to examine student performance on single-subject retests. More specifically, the study objective was to examine whether section retesting results in larger score gains as compared to traditional retesting (taking the entire battery). That is, this study evaluated whether allowing students to take one subject test at a time resulted in students performing better than what is typically seen among students testing with the full battery, taking into account students' prior ACT scores and other testing characteristics. The study also examined how performance on the single-subject retest relates to subsequent performance on the full-battery ACT to evaluate whether any improved performance is associated with true learning gains. Methodology and findings from the study are discussed in detail in this report.