Let X be a Markov process on the line. Under certain conditions it is possible to find a diffusion process which is an approximation to X in the following sense:
(1) X can be embedded in that is there are stopping times (Tt) such that {Xt, t ≥ 0} and have the same distribution;
(2) for each t, E{Tt} = t.
We call the well-timed diffusion approximation to X, and suggest that it is useful for approximating quantities like the first-exit probabilities and expected first-exit times of X.
We determine the well-timed approximation in several special cases and give an asymptotic approximation for use in cases in which cannot be exactly determined.