Since the 2002-2003 academic year, the number of students attending and number of charter schools has grown. Yet, unknown are the comparative performance of charter schools as well as any factors, such as the market-based theory of competition, which may contribute to differences. Using geographic information systems and hierarchical linear modeling, this study examined the achievement scores of 13 charter schools and 172 traditional public schools in the Chicago Public Schools System between academic years 2002-2003 and 2004-2005. The results include significantly positive comparative performance from charter schools but fail to find competition as a contributing factor. (Contains 3 tables and 3 figures.)