COMMUNITIES, STREET GUNS AND HOMICIDE TRAJECTORIES IN CHICAGO, EXAMINING HOMICIDE TRENDS ACROSS SPACE AND TIME* 1980-1995: MERGING METHODS FOR ELIZABETH GRIFFITHS University of Toronto JORGE M. CHAVEZ University at Albany KEYWORDS: communities, space, time, homicide, trajectories zyx We merge Exploratory Spatial Data Analysis (ESDA) and a semi- parametric, group-based trajectory procedure (TRAJ) to classify communities in Chicago by violence trajectories across space. Total, street gun and other weapon homicide trajectories are identified across 831 census tracts between 1980 and zyxw 1995. We find evidence consistent with a weapon substitution effect in violent neighborhoods that are proximate to one another, a defensive diffusion effect of exclusively street gun-specific homicide increases in neighborhoods bordering the zy * Both authors are former pre-doctoral fellows of the National Consortium on Violence Research. This material is based upon work supported by the National Science Foundation under Grant No. SES-0215551 to the National Consortium on Violence Research. Any opinions, findings, and conclusions or recommendations exprcssed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. The initial trajectory analysis portion of this paper was presented at the 2002 NCOVR summer workshop in zyx St. Augustine, FL. An earlier draft of this paper received zyxwv Znd place in the 2003 American Society of Criminology Gene Carte Student Paper competition and was presented at the 2003 ASC conference in Denver, CO. We would like to thank Rosemary Gartner, Breda McCabe, David McDowall, Daniel Nagin, George Tita and anonymous reviewers for their feedback on this manuscript. Please direct all correspondence concerning this article to Elizabeth Griffiths, Department of Sociology and Centre of Criminology, University of Toronto, 725 Spadina Ave, Toronto Ontario, MSS 254, Canada: email: zyxwvut egriffit@chass.utoronto.ca. CRIMINOLOGY VOLUME 42 NUMBER 4 2004 941