Measuring the accuracy of biometrics systems is important. Accuracy estimates depend very much on the quality of the test data that are used. Including poor quality data will degrade the accuracy estimates. What are the good quality data and what are the poor quality data is not revealed by simple accuracy estimates. We propose a novel methodology to analyze how the overall accuracy estimate of a system relates to the specific quality of biometrics samples. Using a large collection of fingerprint samples, we present an analysis of system accuracy, which suggests that a signifi cant part of the error is due to few fingers.