Abstract: A method and apparatus for comparing substances against other substances or against themselves over time. The method involves taking a chromatogram from each substance and converting each chromatogram to a histogram. The histograms have one bar for each peak on the chromatogram. These histograms are then cross-correlated and the 0th, 1st, 2nd and 3rd moments of the cross-correlation function are calculated. A similarity number in terms of the number of peaks of difference between the two samples compared is then generated by dividing the 3rd moment by the 2nd moment, and the results are displayed. The apparatus of the invention provides the means to gather the data needed for the chromatograms, calculate the histograms, cross-correlation functions and moments and the similarity number and display the result.