1. A method of analyzing characteristics of electrocardiographic (ECG) signal data, the method comprising the steps of: obtaining ECG signal data comprising a plurality of waveforms, each waveform comprising a first feature and a second feature, wherein the first feature is a depolarization feature; extracting, with a processor, the depolarization feature from each of the plurality of waveforms; calculating a standard deviation between a plurality of the extracted depolarization features; calculating a dispersion between a plurality of the extracted depolarization features; selecting, with the processor, a set of stable waveforms from the plurality of waveforms that minimizes the calculated standard deviation and minimizes the calculated dispersion; extracting, with the processor, the second feature from each of the waveforms of the set of stable waveforms; and computing, with the processor, a diagnostically relevant value from the extracted second features.