  A new model, called "Human Dynamics", has been recently proposed that
individuals execute activities based on a perceived priority of tasks, which
can be characterized by a power-law distribution of waiting time between
consecutive tasks (Barabasi, 2005). This power-law distribution has been found
to exist in diverse human behaviors, such as mail correspondence, e-mail
communication, webpage browsing, video-on-demand, and mobile phone calls.
However, the pattern has been observed at the global (i.e., aggregated) level
without considering individual differences. To guard against ecological
fallacy, it is necessary to test the model at the individual level. The current
study aims to address the following questions: Is the power-law uniform across
individuals? What distribution do individual behaviors follow? We examine these
questions with a client log file of nearly 4,000 Internet users' web browsing
behavior and a server log file of 2,300,000 users' file-sharing behaviors in a
P2P system. The results confirm the human dynamic model at the aggregate-level
both in webpage browsing and P2P usage behavior. We have also found that there
is detectable variability across the individuals in the decaying rate (i.e.,
the exponent gamma) of the power-law distribution, which follows well-known
distributions (i.e., Gaussian, Weibull, and log-normal).
