The US Bureau of the Census had not officially outlined central business districts prior to 1954. Officials of the Bureau did not address themselves seriously to the problem of outlining the central business districts (CBDs) until the early 1950s. The Bureau decided to base its central business district system upon census tracts. According to the US Bureau of the Census, census tracts are small areas into which large cities and metropolitan areas have been divided for statistical purposes. In 1958, Census CBD data were published in Central Business District Statistics bulletins for most United States cities; for 1963 and later, Census CBD data were published in Major Retail Centers (MRCs) bulletins. The information made available for Census CBDs in MRC bulletins has to do with retailing. Robert Reynolds manipulated the percentages of sales found in each Census CBD retail group and, by relating individual Census CBD group percentages to national percentages, derived a specialization index for comparing the CBDs.