Calculation of a Composite DET Curve

The verification performance of biometric systems is normally evaluated using the receiver operating characteristic (ROC) or detection error trade-off (DET) curve. We propose two new ideas for statistical evaluation of biometric systems based on these data. The first is a new way to normalize match score distributions. A normalized match score, t̂, is calculated as a function of the angle from a representation of (FMR, FNMR) values in polar coordinates from some center. This has the advantage that it does not produce counterintuitive results for systems with unusual DET performance. Secondly, building on this normalization we develop a methodology to calculate an average DET curve. Each biometric system is represented in terms of t̂ to allow genuine and impostor distributions to be combined, and an average DET is then calulated from these new distributions. We then show that this method is equivalent to direct averaging of DET data along each angle from the center. This procedure is then applied to data from a study of human matchers of facial images.