A medical device having a sensor for sampling a biological signal, the biological signal representing a signal waveform and forming a waveform vector composed of the biological signal samples, and a memory for storing a least two threshold vectors composed of boundary samples representing at least two boundaries related to the biological signal defining subspaces for the biological signal samples. 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. An evaluation unit connected to the sensor determines a similarity index (ASCI) by comparing each of the biological signal samples of the waveform vector to corresponding boundary samples of the threshold vectors, thus determining to which subspace each biological signal sample belongs to and creating a trichotomized signal vector, and calculating the signed correlation product of two trichotomized signal vectors.