  We propose a hierarchical logistic equation as a model to describe the
dynamical behavior of a penetration rate of a prevalent stuff. In this model, a
memory, how many people who already possess it a person who does not process it
yet met, is considered, which does not exist in the logistic model. As an
application, we apply this model to iPod sales data, and find that this model
can approximate the data much better than the logistic equation.
