[en] First Born approximation (FBA) and continuum-distorted-wave endash eikonal-initial-state (CDW-EIS) approximation are used to calculate the cross section that is triply differential (TDCS) in the energy and angle of the emitted electron and scattered projectile for proton impact ionization of atomic hydrogen. The TDCS shows different prominent features, whose origin is examined on the basis of the results given by FBA calculations and reported for different physical effects. We discuss how these features are related to those present in the doubly differential cross section (DDCS). copyright 1996 The American Physical Society