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Biometric Recognition: Challenges and Opportunities
unenrolled presenters, the prior distribution F(−) depends on list size, and match comparisons may be dependent.
For an example of how linkage of F(p) to list size can change these results, consider a closed set identification system scaled up by enrolling many more users, each of whom interacts with the system daily to obtain workplace access, perhaps in a rapidly expanding corporation. Unless the number of attempted intrusions increases greatly, F(p) is shifted to the right and p stochastically increases. In the resulting change, the increasing dominance of the PPV fraction by its numerator term outweighs the increasing chance of false recognition for any single impostor challenge, because impostor challenges occur with declining relative frequency. Confidence in a match would thereby increase rather than decrease.