The third method for evaluating de novo sequencing tools is to use statistical control of the FDR. This is the standard approach for evaluating database search algorithms: method A is deemed more powerful than method B if, at a fixed FDR threshold of, say, 1%, A detects more peptides than B from the same set of spectra. This approach avoids the need to focus solely on the "identifiable" subset of the observed spectra. However, a significant problem with this approach is that, currently, no one knows how to do it (although at least one recent study claims to have a solution ). Developing methods for de novo FDR control is one of the key outstanding challenges in the field.