 In the 21st century the city has undergone a population growth above that of the national average, and is projected to increase to over 600,000 by 2020.This can largely be attributed to immigration and relatively high birth rates.People from Sheffield (or 'Sheffielders') are colloquially known to residents of the surrounding towns of Barnsley, Doncaster, Rotherham and Chesterfield as "dee-dars", which derives from the traditional pronunciation of the "th" in the dialectal words "thee" and "thou", still used, especially by older people, in South Yorkshire.