Histogram of logarithmic zinc concentration in autistic children. The numbers on the abscissa indicate the logarithms of scalp hair zinc concentrations (ng/g: ppb). The height of each rectangle represents the frequency in the class interval in logarithmic hair zinc level. Two dotted vertical lines represent the −2 and +2 SD level of the reference range of hair zinc concentrations. The histogram of scalp hair zinc concentrations for 2550 children (2108 males and 442 females) aged 0–15 years is shown.