Two distance measures Figures 1 and 2 each show the same twodimensional scatter plot of 20 points, plus one extra point in red. Each point corresponds to a spectrum, in this case measured at only two wavelengths, with one axis for the absorbance at each wavelength: not terribly realistic, but I cannot draw pictures in high dimensions. The blue points are the training data, the red one is a new sample, being checked to see if it belongs to the same population. The cross is the mean of the training spectra, and the circle in Figure 1 and the ellipse in Figure 2 both enclose the region defined as all points no further from the doi: 10.1255/nirn.1167