  Quantum cosmology implies corrections to the classical equations of motion
which may lead to significant departures from the classical trajectory,
especially at high curvature near the big-bang singularity. Corrections could
in principle be significant even in certain low-curvature regimes, provided
that they add up during long cosmic evolution. The analysis of such terms is
therefore an important problem to make sure that the theory shows acceptable
semiclassical behavior. This paper presents a general search for terms of this
type as corrections in effective equations for a k=0 isotropic quantum
cosmological model with a free, massless scalar field. Specifically, the
question of whether such models can show a collapse by quantum effects is
studied, and it turns out that factor-ordering choices in the Hamiltonian
constraint are especially relevant in this regard. A systematic analysis of
factor-ordering ambiguities in effective equations is therefore developed.
