United Nations Children's Fund (UNICEF). 2009. Statistics and Monitoring. New York: UNICEF. Available online at: http://www.unicef.org/statistics/
Children undernourished is the percent of children under 5 who are moderately or severely underweight. A moderately underweight child is defined as having a weight-for-age ratio that is minus 2 standard deviation from the median, while a severely underweight child is minus 3 standard deviations from the median. The median is derived from an international reference population defined by the World Health Organization.
The source of the data comes from several household surveys developed and implemented by UNICEF as well as official national data reported to WHO.