Top: computer time required for FORD to classify subsets of the SDSS-DR7 galaxy spectrum catalog. Given the number of galaxies to be classified (in abscissa), the time when using k-means (the solid line) is systematically longer than the time for the alternative single pass k-means (the dashed line). The computer time increases linearly with the number of galaxies in the catalog. Error bars code the dispersion produced by the random initialization of the algorithms. Bottom: gain when using single pass k-means, which saves between 20% and 40% of the time.
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