  Arrival times of requests to print in a student laboratory were analyzed.
Inter-arrival times between subsequent requests follow a universal scaling law
relating time intervals and the size of the request, indicating a scale
invariant dynamics with respect to the size. The cumulative distribution of
file sizes is well-described by a modified power law often seen in
non-equilibrium critical systems. For each user, waiting times between their
individual requests show long range dependence and are broadly distributed from
seconds to weeks. All results are incompatible with Poisson models, and may
provide evidence of critical dynamics associated with voluntary thought
processes in the brain.
