The research team used the Global Terrorism Database (GTD) to create new, quantitative measures detailing the use of Improvised Explosive Devices (IEDs) by terrorists, worldwide, from 1970 to 2004. We define IEDs as: Bombs that are constructed in part or wholly from military or commercial explosives or commercial components, and used in a manner other than intended by the manufacturer.
The world wide use of IEDs by terrorists has increased over time, but generally follows trends in other types of terrorist activity. The proportion of terrorist attacks that depend on IEDs has remained relatively steady from 1970 to 2004. IEDs are used in a majority of all explosive attacks. As in other types of terrorist attacks, Western Europe, South America, and the Middle East & North African regions have experienced the highest prevalence of IED attacks over the time period spanned by the data. The use of IEDs is not a good predictor of death or injury from an IED attack; that is, the use of IEDs is unrelated to overall death rates caused by terrorist attacks. The use of IEDs by terrorists in the United States makes up a very small proportion of worldwide terrorist attacks and this proportion has decreased over the period of this study. Because there are few IED attacks in the United States by terrorists, interpretations of the characteristics of these attacks can be strongly affected by a few large-scale incidents. The use of suicide IEDs has grown exponentially over the period studied. The specialized use of VBIEDs and suicide IEDs results in a much higher rate of fatality per attack than the use of other types of IEDs.
The project used existing START data, specifically the Global Terrorism Database (GTD) and the Minorities at Risk Organizational Behavior Database (MAROB) to employ a variety of quantitative techniques. Methods such as such as correlation and regression were used to describe and determine the covariates of IED usage such as location, time, target type, group identity, fatalities and injuries. Using the covariates of IED attacks as a basis for continued analyses it was appropriate to use trajectory analysis to determine whether unique attack patterns emerged over time. These methods were especially useful for detecting either rapid increases or decreases in terrorist attacks using IEDs. It was also possible to identify one or more case studies that allowed us to determine the efficacy of interventions countering IED's through the use of Cox hazard modeling. Finally, the MAROB data was used to compare ethno-nationalist groups in the Middle East that use IED attacks to those that use other violent means and those that do not use violence. In this case, categorical regression models were employed to better understand the relationships between group types, the number of attacks, whether or not the groups attack, the means of attack, and the lethality of attacks.
The major methodological challenge faced by this project was to determine which incidents from the Global Terrorism Database (GTD) involved the terrorist use of an IED based on the IED definition given above. We reviewed and classified 66,509 terrorist attacks for IED involvement in 205 countries between 1970 and 2004. Cases were assessed using a standardized coding scheme across all years and incidents. We assigned a portion of the cases to multiple coders so that we could estimate coding reliability statistics and ensure that the separate coding efforts were measuring IEDs in the data systematically and that the resulting codes from separate coders were reliable, valid, and could be compared across coding samples. Approximately 12,000 cases in the GTD1.1 were not coded because weapon details were unavailable for those cases. As this information becomes available in the future, we recommend that these cases be added to the current IED data base. After the coding scheme was finalized, data collected, and validity assessed, a variety of empirical analyses of the data were conducted. These analyses are intended to provide an empirical baseline of IED terrorist attacks from around the world. This research was supported by the DHS Science and Technology Directorate's Human Factors/Behavioral Sciences Division.