The present invention is a system and method that produces features and indices that indicate the presence or absence of a disease or condition, or of the progression of a disease or condition. The system and method of the present invention also produce features and indices that predict responsiveness to medication from a premedication baseline. The system and method of the present invention further incorporates a testing methodology to improve the performance characteristics of the features or indices. To obtain such features and indices, time domain, power spectrum, bispectrum and higher order spectrum values are derived from biopotential signals taken from the subject being tested.