The Global Terrorism Database--or GTD--began in 2001 when researchers at the University of Maryland obtained a large database originally collected by the Pinkerton Global Intelligence Services (PGIS).
From 1970 to 1997, PGIS trained researchers--mostly retired Air Force personnel--to identify and record terrorism incidents from wire services, government reports, and major international newspapers in order to assess the risk of terrorism for their clients. With funding from the National Institute of Justice, the Maryland team finished digitizing the original Pinkerton data in December 2005, making corrections and adding additional information wherever possible. PGIS lost data for 1993 in an office move and these data have never been fully recovered.
In April 2006, the National Consortium for the Study of Terrorism and Responses to Terrorism (START), working with the Center for Terrorism and Intelligence Studies (CETIS), received additional funding from the Human Factors Division of the Department of Homeland Security (DHS) to extend the GTD beyond 1997. This effort is distinct from the original collection because CETIS analysts had to search archival sources for documented attacks, rather than recording events as they occurred. Some earlier media sources are simply unavailable, undoubtedly reducing the total number of attacks that were identified since 1997. In addition, efforts begun for post-1997 cases used a slightly modified definition of terrorism, with information on specific criteria for identification as a terrorist incident included for each case, and expanded the number of variables collected for each attack.
By August 2008 data collection was completed for incidents that occurred through 2007. In Fall 2008, the GTD team applied the newly-developed inclusion criteria to the earlier GTD data in order to form a single source of information on terrorist attacks, covering the entire period 1970 to 2007 (see the "Synthesis" section of Data Collection and Methodology).
In Spring 2008, analysts from the Institute for the Study of Violent Groups (ISVG) at the University of New Haven collected data for the GTD on terrorist attacks that occurred between April 2008 and October 2011. Research assistants at START at the University of Maryland integrated these data into the database, while continuing to make improvements to the earlier data. GTD staff members at START routinely review a wide array of sources to identify additional cases and additional information about previously identified cases, to help ensure that GTD is as comprehensive and accurate as possible for its full time span. Data from these collection efforts are also integrated into the current version of GTD. Additional information on these collection efforts can be found in the codebook (see dbsource field). Beginning with attacks that occurred in November 2011, all ongoing GTD data collection efforts are conducted by START staff at the University of Maryland.
Thus, the Global Terrorism Database is a compilation of distinct data collection efforts from 1970 to the present. From 1970 to 1997 the data were constructed primarily from incidents recorded in real-time by PGIS using a broad-based definition of terrorism. Data from this period are updated and corrected on an ongoing basis. The data from 1998 through 2007 were primarily collected retrospectively, with data on more recent events being collected in real-time and with the benefit of more robust media archives. Thus, users should note that differences in levels of attacks before and after 1997 may be at least partially explained by differences in data collection; researchers should adjust for these differences when modeling the data. Nonetheless, criteria for collecting data were established and applied to the full set of cases to assure the adherence to a broad definition of terrorism and also to allow users to filter out cases that may be inappropriate for their specific analytical interests. The data now form a complete series from 1970 to the present--except for 1993.