or clandestine agents, usually intended to influence an audience”; while for instance, Krueger and Maleckova (2002) adopt the following one: ”the premeditated use, or threat of use, of extreme violence to obtain a political objective through intimidation or fear directed at a large audience”. The term ”transnational terrorism” applies to terrorist acts involving citizens or the territory of more than one country, either as victims or perpetrators.

One of the most exhaustive definitions of transnational terrorism is the one by the ITERATE database which defines terrorism acts as ”the use, or threat of use, of anxiety-inducing, extra-normal violence for political purposes by any individual or group, whether acting for or in opposition to established governmental authority, when such action is intended to influence the attitudes and behavior of a target group wider than the immediate victims and when, through the nationality or foreign ties of its perpetrators, its location, the nature of its institutional or human victims, or the mechanics of its resolution, its ramifications transcend national boundaries”.

Whatever the definition, three characteristics are essential to all of them: a) the terrorist act is premeditated and politically motivated, b) it is done with the idea to intimidate an audience larger than the immediate victims, c) and is undertaken by groups which have no sovereign national recognition.<sup>3</sup> A frequently used indicator to measure terrorism is the number of terrorist incidents. As mentioned by Frey, Luechinger and Stutzer (2004), counting the number of terrorist incidents means that one cannot differentiate between terrorist activities of different magnitudes. One way though to mitigate this problem is to use the number of casualties (killed or injured people). Another problem of most datasets on terrorist events is the reporting problem. Only events reported in official statistics and the media are counted. This may introduce a bias because governments may not know or may deliberately not report some events. For obvious reasons, this last fact is more likely to happen for authoritarian countries where the freedom of information is limited. As well, the media may only pick up events with high visibility (for instance in big cities, creating an anti-rural bias in this case).

In the case of transnational terrorism, these reporting biases are likely to be less severe as information may be revealed from diverse sources. One of the most frequently used databases on transnational terrorism is ITERATE (International Terrorism: Attributes of Terrorist Events) collected by Mickolus et al. (1989,1993, 2003). It compiles publicly available media sources on transnational terrorist incidents and delivers various information on the country of location of each incident, the number and nationality of the victims, the nationality of the terrorist organization (when reported), the type of incident (high-jacking, bombings, hostage taking, etc...). Enders and Sandler (2006) provide a complete discussion of the database and document several empirical facts:

> 3Another element stressed for instance by Omar Malik (2003) from the Royal Institute of International Affairs is the fact that only those incidents that are perpetrated against or within _liberal_ states should be qualified as terrorist attacks. A country is said to be liberal when it safeguards human rights in its laws and practices. Qualifying terrorism acts the rest of the incidents against non-liberal countries is usually more controversial. For some observers, these actions might be viewed as terrorism but for others, they might be rather qualified as acts of resistance against a totalitarian country.

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