In the case of synthetic O-D generation, the desire is to select from among all of the possible solutions, the most likely.
This approach requires one to define a measure of the likelihood of each matrix. In general, there are two approaches to
establish the likelihood of a matrix. One of them treats the trip as the basic unit of observation, while the other considers a
volume count as the basic unit of observation. Both approaches will be discussed in greater detail later, but for now it
suffices to indicate that for any matrix with cells Tij, the likelihood of the matrix can be estimated using a function L=f(Tij,tij),
where tij represents prior information. The prior information is often referred to as a seed matrix, and can be derived from
a previous study or survey. In the absence of such prior information, all of the cells in this prior matrix should be set to a
uniform set of values.