Statistics Iceland began publishing figures on migration in 1961, before which the main source of information on migration was censuses. Until the end of 1985 figures on migration were processed once per year, thus all persons who moved residence over the course of the year were only counted as moving once. It was measured based on change of residence from December of the previous year to December of the current year. With the founding of the Register of Migration Data in 1986 figures on migration were processed monthly, based on individuals having to be resident for at least one month in the same place before being considered as having moved.