For many family history researchers the 1900 U.S. Census is of tremedous help. That is the first Census that gives the year of immigration, which can be very helpful in getting ones ancestor back to the "old country".
Emigration records from Norway start 1867. Therefore these U.S. Census (1850 & 1860) records can be of great help in identifying ones Norwegian ancestors already here in America. Although the Census records do not usually give the place of birth in Norway, many give an additional "last name" or the farm name in Norway making it possible to trace a person back to Norway, and a person's age.