This study examines the relationship between student mobility and achievement test results in predominately Black and Hispanic Community School District 17, Brooklyn, New York. The project is designed to assess the impact of mobility on pupil achievement and the interpretation of achievement data, as well as to map the pupil population mobility. A second purpose is development of a replicable procedure for mapping in an urban school district. The related literature was reviewed to help define the study. Reading and mathematics scores of 2944 third-, 2997 fourth- and 2877 fifth-grade students, number of moves, and 13 other variables were collected. These data were examined to determine whether year one and year two test results involve the same populations, whether there are differences in the California Achievement Reading Test results and Standardized Diagnostic Mathematics Test results of non-mobile students, and whether there is a relationship among other variables contributing to achievement and test results. The Statistical Package for Social Sciences (SPSS) was used to analyze intra-mobility and outward mobility as independent variables and achievement as the dependent variable. It was found that district schools are not looking at the same population year after year. Therefore the emphasis on test score utilization for instructional planning is invalid. (CM)