mance of one component sometimes degrades the performance of a larger system in which it resides.23 There is no way to definitively determine the impact of component changes on system-level performance until the components have been inserted and the system is tested. System performance may depend critically on factors such as the current performance levels of other components, whether the performance characteristics of some components depend upon the outputs of other components, the nature and quality of system input material, and the characteristics of the environment in which the system operates.


Upgrading a face image detector in a facial recognition system is one example of how improving performance of a component can degrade the system. If the system can find more faces in more poses, then the number of off-angle and partial faces sent into the comparison process increases. This suboptimal data situation can lead to poor performance. Moving from lossy to loss-less compression is another example where a local improvement can degrade overall system effectiveness. See, for example, NIST, Effect of image size and compression on one-to-one fingerprint matching, NISTIR 7201 (February 2005), and J. Daugman and C. Downing, Effect of severe image compression on iris recognition performance, IEEE Transactions on Information Forensics and Security 3:1 (2008).

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