Both the synthetic O-D generation process and the trip distribution process are subject to an information underspecification
limitation. For example, if one attempts to estimate an O-D matrix for a 100 zone network with, say 1000
links, one has more unknowns to solve for than there are constraints. In the case of the trip distribution process, there are
100x100 O-D cells to be estimate, and only 2x100 trip end constraints. In the case of the synthetic O-D generation
process, there are again 100x100 O-D cells to estimate, and only 1000 link constraints. Given the possibility of multiple
solutions, both the trip distribution process and the synthetic O-D generation process invoke additional considerations to
select a preferred matrix from among the multiple solutions, as indicated next.