In the reporting of individual student results from standardized tests in educational assessments, the percentile rank of the individual student is a major numerical indicator. For example, in the 1998 and 1999 California Standardized Testing and Reporting (STAR) program using the Stanford Achievement Test Series, Ninth Edition, Form T (Stanford 9), the 1998 "Home Report" and the 1999 "Parent Report" feature solely the National Grade Percentile Ranks. (These percentile rank scores are also featured in the more extensive "Student Report.") This paper develops a formulation and presents calculations to examine the properties of year-1, year-2 comparisons using these individual percentile rank scores. The approach and formulation follow the previous investigations of the accuracy of the individual percentile rank score in D. Rogosa (1999). A typical question that this paper addresses is: What are the chances that a student who really improved 10 percentile points from year-1 (1998) to year-2 (1999) obtains a lower percentile rank in year-2 than year-1? Such questions are addressed using the test reliability coefficient in classical test theory to represent the quality of measurement. These approaches make it possible to investigate the question, What level of test reliability is needed to obtain good accuracy in year-1, year-2 comparisons? (Author/SLD)