@inbook{NBERc5113,
 Crossref = "NBERwill31-1",
 title = "Migration Statistics of Denmark, Norway and Sweden",
 author = "Adolph Jensen",
 BookTitle = "International Migrations, Volume II: Interpretations",
 Publisher = "NBER",
 pages = "283-312",
 year = "1931",
 URL = "http://www.nber.org/chapters/c5113",
}
@book{NBERwill31-1,
 title = "International Migrations, Volume II: Interpretations",
 author = "Walter F. Willcox",
 institution = "National Bureau of Economic Research",
 type = "Book",
 publisher = "NBER",
 year = "1931",
 URL = "http://www.nber.org/books/will31-1",
}