Histograms of the zinc/iron ratio in autistic children. The histogram of the zinc/iron ratio for 2550 children is shown in the logarithm. The numbers on the abscissa indicate the logarithms of the zinc/iron ratio. The height of each rectangle represents the frequency in the class interval in the logarithmic zinc/iron ratio. The dotted vertical lines represent the –2 SD level of the reference range of hair zinc/iron ratio.