Read two biometrics get worse results

John Lettice at the Register has been going over some of John Daugman's calculations about the effects of combining multiple biometrics to authenticate an individual. The results are actually quite straightforward when you stop to think about it, though not necessarily intuitive.

"On the one hand, a combination of different tests should improve performance, because more information is better than less information. But on the other, the combination of a strong test with a weak test to an extent averages the result, so the result should be less reliable than if one were relying solely on the strong test...

If the two biometric tests differ significantly in their power, and each operates at its own cross-over point, then combining them gives significantly worse performance than relying solely on the stronger biometric...

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...

Which suggests to us that simply regarding a second or third biometric as a fall back to be used only if earlier tests fail constructs a scenario where the combined results will be worse than use of the single stronger test..."