The lowest fully tiled Census geography above VTD is the MCD. Hence the ROAD Project sought to aggregate precincts to MCDs and then merge the Census data with the precinct-level electoral data files. This was possible for a number of states--primarily the Northeast plus a few from the Midwest--but the precinct-level electoral data files for the vast majority of states do not contain MCD identifiers and do not neatly nest VTDs within MCDs.
This problem is illustrated nicely in an attached figure. This figure shows a map of Walker County, Georgia. It also shows the Census VTD and MCD boundaries, with thin red and wide gray lines, respectively. Notice how the two VTDs in the lower right hand corner of the county neatly fit within one MCD. Compare this to the top portion of the map, in which at least three VTDs are not fully contained within single MCDs.
Figure 4.1: 1990 Census Geography in Walker County, Georgia
Because of this problem, we could not merge properly at the MCD level for the majority of states. We would have had to arbitrarily assign an overlapping VTD to a single one of its MCDs. In Walker County, for instance, 3 of 8 of the MCDs would have been shorn of at least half their area (and presumably a similar proportion of population), and the other MCDs would have also inappropriately absorbed the excess. The population from which the voting data was drawn would simply not match the population on which the Census data was gathered.
Thus, in order to merge these two sets of data, we had to create an entirely new unit of analysis. This unit is the smallest possible aggregation of MCDs such that no VTD within its borders is also a member of any other such aggregate. We call this unit the MCD Group. Since no VTD or MCD ever crosses county lines, an MCD Group can be no larger than a county and no smaller than a single MCD.
The figure illustrates how MCD Groups relate to MCDs and VTDs in Walker County. Recall how the VTDs in the top portion of the map overlap MCDs. All the MCDs linked by a chain of overlapping VTDs are members of the same MCD Group, the large central area described by the vertical light gray stripes. Yet those MCDs not sharing VTDs with others constitute their own MCD Groups, as you can see in the lower right hand corner of the map, for example. In total, Walker County has 17 VTDs, 8 MCDs, and 4 MCD Groups.
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