"This report presents new information on the long-term trend in street gang activity and violent crime in the United States. The major focus of the trend analyses reported here is on the ebb and flow of gang activity in U.S. cities and counties of varying sizes. For the first time, trajectory analysis, which can group cities according to common patterns, is used to examine cities' and other localities' histories of gang problems as a way of gaining insights into gang activity across multiple years. In the second section of this report, attention is turned to large cities' violent gang histories."
Natrional Gang Center: http://www.nationalgangcenter.gov/
National Gang Center Bulletin (May 2011), No. 6