To formalize this somewhat consider the waiting line as a system S into which there is a flow of particles (customers) and where the process “buy ticket” means that the particle leaves the system. The waiting time we have considered above is commonly referred to as transit time, and the theorem we have applied is occasionally called the Little's theorem, which could be formulated as: the expected steady state number of particles in the system S equals the flow of particles into S times the mean transit time. Similar theorems have been discovered in other fields, and in physiology it was earlier known as one of the Stewart-Hamilton equations (e.g. used for estimation of blood volume of organs).