Schools and libraries receive hundreds of challenges to books each year in the United States. According to the Office of Intellectual Freedom (OIF) of the American Library Association (ALA), there were at least 326 challenges in 2011. The ALA estimates that "for every reported challenge, four or five remain unreported" (Frequently challenged books of the 21st century).
This map was created for the BannedBooksWeek.org site from information provided by the ALA and the Kids' Right to Read Project (which is a collaboration of the American Booksellers Foundation for Free Expression and the National Coalition Against Censorship). The map creators gathered data from ALA reports ("Books Banned and Challenged 2007-2008; 2008-2009; 2009-2010; and 2010-2011") as well as the "Kids' Right to Read Project Report."