Robust dynamic classes revealed by measuring the response function of a social system

From Riley Crane and Didier Sornette:

We find that most videos’ dynamics (≈90%) either do not experience much activity or can be described statistically as a Poisson process (verified using a Chi-Squared test). This large set of data is consistent with the endo-subcritical classification. For the remaining 10% (≈500, 000 videos), we find nontrivial herding behavior that accurately obeys the three power-law relations described above. Characteristic examples of endogenous and exogenous dynamics are shown in Fig. 3.

Via Computational Legal Studies blog.

By what mechanism does traffic transition form Poisson to Power-law statistics? Click on the "syncronization" category at on the right to see some speculations. It seems that spatially dispersed clusters are syncing up. In the exogenous case, that's in response to a driving force -- new information. In the endogenous case, the mechanism is more subtle, and may be an example of self-organization. See the Zanette and Kuperman summary especially.