Can the average drift of an ensemble of charged particles in Earth's plasma sheet still be described by adiabatic theory even if many particles in the ensemble execute non adiabatic motion? This is part of a broader spectrum of questions which asks if chaotic microscopic processes can be parameterized as macroscopic ones when ensemble averaged. Through a comparison of numerical test particle simulations with adiabatic theory we show that the average particle drift speed of an appropriately chosen ensemble of nonrelativistic particles, including those executing chaotic and Speiser [1965, 1967> motion, is given correctly by the simple adiabatic guiding-center drift formula. We further show that the dispersion of particles about the mean drift speed tends to decrease due to the presence of chaotic particle scattering. These conclusions are demonstrated for tail-like magnetic fields B(x,z), including a non zero y component. The presence of an electric field does not change the conclusions provided that E/B is small compared to the total velocity of an individual particle. Thus we have shown that a standard way of representing particle transport in the middle magnetosphere, namely, the formalism describing average drift in a flux tube filled with an isotropic particle distribution, remains a useful theoretical description for the central plasma sheet, despite the presence of non adiabatic particle motion. ¿ American Geophysical Union 1996