The techniques of modulated and transient photoconductivity provide useful information about the density of traps and defects in the band gap of semiconductors like hydrogenated amorphous silicon. microcrystalline silicon, copper indium diselenide and many others by application of appropriate equations to convert the measured data to a density of states. The paper discusses two cases that are relevant for photovoltaic materials: a discrete set of trap states in the band gap and a continuous distribution. Partial occupancy of the states is achieved by the position of the Fermi energy or the quasi-Fermi energy which results in a limitation of the validity of the analysis. We discuss the implications for the interpretation of the reconstructed density-of-states profile. Simulation results are also compared with experimental data with emphasis on n-type hydrogenated amorphous silicon.