Depending on the order up to which the moment equations are taken into account, transport
models of different levels of sophistication are obtained. A characteristic of the moments
method is that each equation for the moment of the distribution function contains the next
higher moment . For example eqn. (2.104) contains the sixth order moment
. Therefore the number of unknowns exceeds the number of equations and an
expression for the highest occurring moment must be found. This can be achieved either by
simplifying the equation of order or by invoking some physical reasoning independent
of the derivation of the moments themselves. This task is referred to as closure of
the moment equations.