  A partial differential equation model is analyzed for the two-slit experiment
of quantum mechanics. The state variable of the equation is the probability
density function of particle positions. The equation has a diffusion term
corresponding to the random movement of particles, and a nonlocal advection
term corresponding to the movement of particles in the transverse direction
perpendicular to their forward movement. The model is compared to the
Schr\"odinger equation model of the experiment. The model supports the ensemble
interpretation of quantum mechanics.
