1. A method comprising: (A) applying a first filter to a data vector and a set R, the set R containing at least one reference vector, to produce first filter output representing a measure of similarity of the data vector to at least one reference vector in the set R; (B) applying a second filter to second filter inputs to produce second filter output, wherein the first filter has at least one failure mode not shared by the second filter; (C) deriving, from the first filter output and the second filter output, confidence estimate output representing a degree of confidence in the correctness of the first filter output.