Percentage of children under five that are underweight = (Number of children under age five that fall below minus two standard deviations from the median weight for age of the NCHS/WHO standard (moderate and severe))*100/ Total number of children under age five that were weighted. The NCHS/WHO reference standard represents the distribution of height and weight by age and sex in a well-nourished population. In a well-nourished population, 2.3 percent of children fall below minus two standard deviations.