In this study we investigate two distinct loss mechanisms responsible for the rapid dropouts of radiation belt electrons by assimilating data from Van Allen Probes A and B and Geostationary Operational Environmental Satellites (GOES) 13 and 15 into a 3-D diffusion model. In particular, we examine the respective contribution of electromagnetic ion cyclotron (EMIC) wave scattering and magnetopause shadowing for values of the first adiabatic invariant μ ranging from 300 to 3000 MeV G. We inspect the innovation vector and perform a statistical analysis to quantitatively assess the effect of both processes as a function of various geomagnetic indices, solar wind parameters, and radial distance from the Earth. Our results are in agreement with previous studies that demonstrated the energy dependence of these two mechanisms. Loss from L* = 4 to L* = 4.8 is dominated by EMIC wave scattering (μ ≥ 900 MeV G) and may amount to between 10%/hr to 30%/hr of the maximum value of phase space density (PSD) over all L shells for fixed first and second adiabatic invariants. Magnetopause shadowing is shown to deplete electrons across all energies, mostly between L* = 5 and L* = 6.6, resulting in loss from 50%/hr to 70%/hr of the maximum PSD. We also identify a boundary located between L* = 3.5 and L* = 5.2 clearly separating the regions where each mechanism dominates. Nevertheless, during times of enhanced geomagnetic activity, both processes can operate beyond such location and encompass the entire outer radiation belt.