Hennessy A. Christopher and Toni M.
Whited (2005) argued that traditional formulations of the financing decision
place the firm at date zero with no cash on hand. Such firms are at the debt
versus external equity financing margin, since each dollar of debt replaces a
dollar of external equity. The problem with the traditional approach is that
corporations do not spend their lives at date zero. Rather, they evolve in a
stochastic way, finding themselves at different financing margins over time.