There is a central irony in our approach to global terrorism: while effective policy against terrorism depends especially on hard data and objective analysis, the study of terrorism has lagged behind many other fields in the social and behavioral sciences. In their encyclopedic review of political terrorism, Schmid and Jongman identified more than 6,000 published works but point out that much of the research is "impressionistic, superficial [and offers]... far reaching generalizations on the basis of episodal evidence."