When it is impractical to rigorously assess all parts of complex systems, test engineers use defect detectors to focus their limited resources. In this article, we define some properties of an ideal defect detector and assess different methods of generating one. In the case study presented here, traditional methods of generating such detectors (e.g. reusing detectors from the literature, linear regression, model trees) were found to be inferior to those found via a PACE analysis.