Cycles and Substitutions in Terrorist Activities: A Spectral Approach
使用谱分析发现恐怖事件、劫机、绑架等序列存在显著周期（如28个月），并通过交叉谱分析揭示恐怖分子在短期内在不同事件类型间存在替代行为。
SUMMARY We use spectral analysis to provide evidence that cycles have characterized the following time series: all terrorist events, skyjackings, kidnappings, barricade and hostage‐taking, and all events not involving hostages. The series for all events had a periodicity of 28 months, while skyjackings had two significant periodicities ‐ 4.1 months and 28 months. Only a single significant periodicity was associated with barricade and hostage events and kidnappings ‐72 months and 48 months, respectively. We also found that nonlinear trends best represented four of the five series. Cross‐spectral analysis was then applied to the six possible pairs of series; each pair displays from three to nine statistically significant coherencies. Moreover, we discovered evidence of orthogonality for five of the six pairs studied. This evidence suggests that terrorists substitute between related events; this substitution primarily showed up in the short run.