From Osher Doctorow

I should emphasize that X and S, or x and s as values of them
respectively, are roughly speaking frequency versus intensity of
terrorist events respectively. The authors point out that there is a
well established relationship between the frequency and intensity of
wars, citing 4 papers on this, including 2 by the famous L. F.
Richardson (1948, 1960), one by J. S. Levy (1983), and one by D. C.
Roberts and D. L. Turcotte (1998).

The authors indicate that small groups of non-state actors may have
access to more destructive potential in industrialized nations than
elsewhere.

Osher Doctorow