The changing clusters of terrorism

I've been looking at the data set from the Global Terrorism Database, an impressively detailed register of terrorism events worldwide since 1970. Before delving into the more finer-grained data, the first questions I wanted to ask for my own edification where

  • Is the frequency of terrorism events in different countries correlated?
  • If so, does this correlation changes over time?

What I did was summarize event counts by country and month, segment the data set by decade, and build correlation clusters for the countries with the most events each decade depending on co-occurring event counts.

The '70s looks more or less how you'd expect them to:

cluster1970

The correlation between El Salvador and Guatemala, starting to pick up in the 1980's, is both expected and clear in the data. Colombia and Sri Lanka's correlation is probably acausal, although you could argue for some structural similarities in both conflicts:

cluster1980

I don't understand the 1990's, I confess (on the other hand, I didn't understand them as they happened, either):

cluster1990

The 2000's make more sense (loosely speaking): Afghanistan and Iraq are close, and so are India and Pakistan.

cluster2000

Finally, the 2010's are still ongoing, but the pattern in this graph could be used to organize the international terrorism-related section of a news site:

cluster2010

I find most interesting how the India-Pakistan link of the 2000's has shifted to a Pakistan-Afghanistan-Iraq one. Needless to say, caveat emptor: shallow correlations between small groups of short time series is only one step above throwing bones into the ground and reading the resulting patterns, in terms of analytic reliability and power.

That said, it's possible in principle to use a more detailed data set (ideally, including more than visible, successful events) to understand and talk about international relationships of this kind. In fact, there's quite sophisticated modeling work being done in this area, both academically and in less open venues. It's a fascinating field, and if it might not lead to less violence in any direct way, anything that enhances our understanding of, and our public discourse about, these matters is a good thing.