Estimates of Mexico's white population differ greatly in both, methodology and percentages given, extra-official sources such as the "World Factbook" and "Encyclopædia Britannica", which use the 1921 census results as the base of their estimations, calculate Mexico's white population as only 9% or between one tenth to one fifth (the results of the 1921 census, however, have been contested by various historians and deemed inaccurate). Surveys that account for phenotypical traits and have performed actual field research suggest rather higher percentages: using the presence of blond hair as reference to classify a Mexican as white, the Metropolitan Autonomous University of Mexico calculated the percentage of said ethnic group at 23%. With a similar methodology, the American Sociological Association obtained a percentage of 18.8%. Another study made by the University College London in collaboration with Mexico's National Institute of Anthropology and History found that the frequencies of blond hair and light eyes in Mexicans are of 18% and 28% respectively, nationwide surveys in the general population that use as reference skin color such as those made by Mexico's National Council to Prevent Discrimination and Mexico's National Institute of Statistics and Geography report percentages of 47% and 49% respectively. A study performed in hospitals of Mexico City reported that an average 51.8% of Mexican newborns presented the congenital skin birthmark known as the Mongolian spot whilst it was absent in 48.2% of the analyzed babies. The Mongolian spot appears with a very high frequency (85–95%) in Asian, Native American, and African children. The skin lesion reportedly almost always appears on South American and Mexican children who are racially Mestizos, while having a very low frequency (5–10%) in Caucasian children. According to the Mexican Social Security Institute (shortened as IMSS) nationwide, around half of Mexican babies have the Mongolian spot.