Data reduction. Data reduction by finding peak maxima and combining the measured peptide masses in the different spectra. Occurrences of masses with a window of 0.5 Da are summed, and the average value of the mass is calculated (dashed line). The occurrence of only one peptide in the second spectrum is summed in the mass window of the first spectrum. The second peak of spectrum 2 and not the first one is combined with the first peak of spectrum 1, since it has the closest distance in mass (Da) with the first peak of spectrum 1. Not previously registered masses in the first spectrum are added to the list. The clustering continues iteratively through all mass spectra of the samples in the matrix.