Other sources disagree. Population of Seoul Reference source used Where they got the information Largest City 10,231,217 1. Information Please Almanac online , visited Sept.1999 Lists source as the 1997 Demographic Yearbook, United Nations. Some sources are older or newer sources may have older data. The definition of a city boundary varies in different reports.
Population is only one example.
It's always a good idea to tell users the name and date of the resource you used to answer a question and check more than one source if at all possible. Anybody can put up an Internet site. Anybody can have a book printed for that matter. Someone in your library chooses reference sources that are going to be the most valuable for your patrons. Libraries that are part of the Three Rivers Library System may request certificates of completion for their employees who complete this module.