Read two biometrics, get worse results - how it works | The Register

Read two biometrics, get worse results - how it works | The Register: "Daugman produces the calculations governing the use of two hypothetical biometrics, one with both false accept and false reject rates of one in 100, and the second with the two rates at one in 1,000. On its own, biometric one would produce 2,000 errors in 100,000 tests, while biometric two would produce 200. You can treat the use of two biometrics in one of two ways - the subject must be required to pass both (the 'AND' rule) or the subject need only pass one (the 'OR' rule). Daugman finds that under either rule there would be 1,100 errors, i.e. 5.5 times more errors than if the stronger test were used alone.
He concludes that a stronger biometric is therefore better used alone than in combination, but only when both are operating at their crossover points. If the false accept rate (when using the 'OR' rule) or the false reject rate (when using the 'AND' rule) is brought down sufficiently (to 'smaller than twice the crossover error rate of the stronger test', says Daugman) then use of two can improve results. If we recklessly attempt to put a non-mathemetical gloss on that, we could think of the subject having to pass two tests (in the case of the 'AND') rule of, say, facial and iris. Dropping the false reject rate of the facial test (i.e. letting more people through) in line with Daugman's calculations would produce a better result than using iris alone, but if the facial system rejects fewer people wrongly, then it will presumably be accepting more people wrongly."