Estimates of Mexico's white population differ greatly in both, methodology and percentages given, extra-official sources such as The World Factbook and Encyclopedia 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% having its higher frequency on the North region (22.3%–23.9%) followed by the Center region (18.4%–21.3%) and the South region (11.9%).