The National Archives has microfilm copies of card indexes that can be useful in obtaining United States Federal Census records for the 1880 (partial), 1900,1910 and 1920 censuses.
When using census records, it is recommended that you begin your research in the most recent available indexed census. At present, only federal censuses through 1920 are open to the public. To protect the privacy of the living American people whose names appear in each schedule, population schedules are closed for 72 years after the census is taken, and are not generally available to researchers during that time.
In the late 1930s the federal government,