We repeated the latter experiment using a single frequency
in order to demonstrate the utility of employing a swept
frequency approach. We used attribute selection to identify
753.5kHz as the single best frequency at which to differentiate
users. It should be noted that in a real world system
this ideal frequency depends on the set of users and environmental
conditions, and thus cannot be known a priori –
thus our estimate is idealized. On average, user differentiation
was 87.3% accurate vs. 97.8% when all frequencies
were used, or roughly six times the error rate.