Why are some geographic units combined in a year-specific geographic level?
For confidentiality purposes, microdata records cannot be associated with a geographic unit that has a population of less than 20,000. To ensure that this doesn�t occur, TerraPop merges units with fewer than 20,000 individuals with neighboring units until the resulting unit has more than 20,000 individuals. Units below the population threshold are merged with the neighboring unit that is most similar in population density. This process is called �regionalization.�