  As a major part of the daily operation in an enterprise, purchasing frequency
is of constant change. Recent approaches on the human dynamics can provide some
new insights into the economic behaviors of companies in the supply chain. This
paper captures the attributes of creation times of purchase orders to an
individual vendor, as well as to all vendors, and further investigates whether
they have some kind of dynamics by applying logarithmic binning to the
construction of distribution plot. It's found that the former displays a
power-law distribution with approximate exponent 2.0, while the latter is
fitted by a mixture distribution with both power-law and exponential
characteristics. Obviously, two distinctive characteristics are presented for
the interval time distribution from the perspective of individual dynamics and
group dynamics. Actually, this mixing feature can be attributed to the fitting
deviations as they are negligible for individual dynamics, but those of
different vendors are cumulated and then lead to an exponential factor for
group dynamics. To better describe the mechanism generating the heterogeneity
of purchase order assignment process from the objective company to all its
vendors, a model driven by product life cycle is introduced, and then the
analytical distribution and the simulation result are obtained, which are in
good line with the empirical data.
