1. A medical device comprising: a. at least one sensor for sampling a biological signal, b. an evaluation unit connected to the sensor, wherein the evaluation unit receives samples of the biological signal and forms a waveform vector composed of the biological signal samples, and c. a memory for storing at least two threshold vectors composed of boundary samples, the threshold vectors representing at least two boundaries related to the biological signal samples, the boundaries defining subspaces for the biological signal samples, wherein one threshold vector is an upper threshold vector composed of upper boundary samples and the other threshold vector is a lower threshold vector composed of lower boundary samples, wherein each lower boundary sample is less than or equal to the corresponding upper boundary sample, wherein the evaluation unit determines a similarity index (ASCI) by: (1) comparing each of the biological signal samples of the waveform vector to corresponding boundary samples of the threshold vectors, (2) creating trichotomized signal vectors dependent on the subspaces to which the biological signal samples correspond, and (3) calculating the signed correlation product of two trichotomized signal vectors.