We report a study of the accuracy with which the Urbach energy and defect density of a‐Si:H and a‐Si,Ge:H alloys may be extracted from optical absorption spectra determined by the constant photocurrent method. Surprisingly, a great part of the discrepancy results from evaluation of the CPM spectra, rather than from the normalization of the spectra to an absolute scale of the optical absorption coefficient. We will discuss the sources of error in several existing methods of data analysis and the adaptability of these methods for studying alloys. As well, we propose a new method of analysis, one in which we look at the excess subgap absorption over a range of energies.