"United States Census, 1880," index and images, FamilySearch (https://familysearch.org/pal:/MM9.1.1/MDLS-MJ4 : accessed 18 Jan 2014), Henry Bierma in household of Sake Keuning, Summit, Marion, Iowa, United States; citing sheet 500A, family 2, NARA microfilm publication T9-0354