  Humans are heterogenous and the behaviors of individuals could be different
from that at the population level. We conduct an in-depth study of the temporal
patterns of cellphone conversation activities of 73'339 anonymous cellphone
users with the same truncated Weibull distribution of inter-call durations. We
find that the individual call events exhibit a pattern of bursts, in which high
activity periods are alternated with low activity periods. Surprisingly, the
number of events in high activity periods are found to conform to a power-law
distribution at the population level, but follow an exponential distribution at
the individual level, which is a hallmark of absence of memory in individual
call activities. Such exponential distribution is also observed for the number
of events in low activity periods. Together with the exponential distributions
of inter-call durations within bursts and of the intervals between consecutive
bursts, we demonstrate that the individual call activities are driven by two
independent Poisson processes, which can be combined within a minimal model in
terms of a two-state first-order Markov chain giving very good agreement with
the empirical distributions using the parameters estimated from real data for
about half of the individuals in our sample. By measuring directly the
distributions of call rates across the population, which exhibit power-law
tails, we explain the difference with previous population level studies,
purporting the existence of power-law distributions, via the "Superposition of
Distributions" mechanism: The superposition of many exponential distributions
of activities with a power-law distribution of their characteristic scales
leads to a power-law distribution of the activities at the population level.
