[en] Differential cross sections have been measured using coplanar kinematics for the electron impact ionization of atomic hydrogen at incident energies of 100eV, 113.6eV, 250eV and 413.6eV for a variety of emitted electron energies and angles. The data are compared with differential cross sections calculated using: (1) the factorized distorted wave half off-shell impulse approximation (DWIA), (2) the plane wave impulse approximation PWIA, and (3) the plane wave Born approximation with exchange (PWBE). The optical model potential used to calculate the distorted waves is energy dependent and has polarization and exchange terms as well as an absorptive part. Various ways of treating the two outgoing electrons are examined in the DWIA