A k value represents a type of summary statistic by means of which glass evidence may be interpreted. This statistic is defined as k = V/D, where the dispersion V = (n_D − 1)/(n_F − n_C), and where D is the density. The k value is therefore the slope of a line through the 0,0 origin and a plot of dispersion versus density. Obtaining a k value facilitates the interpretation of commonality of source, since “within-item” variation in density and refractive index does not significantly alter the k value. “Between-item” variation, on the other hand, profoundly influences the k value, even if the density and one of the refractive indices are coincidentally similar.