Election Night Projections

Precinct Sample

Like all estimation, the election night projections start with the sample. In each state where a projection is desired, a sample of voting precincts must be taken. Depending on the size and diversity of a state, a sample of 60 to 120 precincts is taken as the basis of a projection. This procedure for sampling is known as a stratified systematic sample proportionate to size.

Each state is broken into three to five geographic areas (geo-strata) based on their historical voting patterns. For example, Figure 1 shows the geographic areas (strata) for New Jersey, while Figure 2 shows an example of New Jersey's historical voting patterns.

Figure 1 Example of state-level geo-strata (New Jersey)

A recent past general election in a state is used as the base race for selection for that state. There are three important attributes in choosing the base race: (1) being recent, so as to minimize the geographic boundary changes that may occur from election to [Page 224]election; (2) being competitive, so that it shows a reasonable distribution of the vote; and (3) being typical, in that it reflects the ideology of the political parties that the candidates represent.

Figure 2 Example of historical voting patterns (New Jersey)

A listing of all precincts and vote counts in that past election is obtained and geographic codes are added. The precincts are sorted by geographic area and placed in order of total vote. The precincts in each area are then broken down into two separate size groups (usually at the median of accumulated total vote in the geographic strata). The two size groups are used to minimize the bias that can occur when sampling probability proportionate to size. The precincts in each group within a geographic [Page 225]area are ordered by their percentage of Democratic vote in the base race. A sample of precincts is then selected from each group proportionate to the size of their total vote in the base race with an exception in only one state. There are a couple of precincts in New Hampshire that are larger than the interval of selection, and in those cases the precincts are selected with probability of one (1.0). This procedure gives an excellent probability sample of the state for the past race.

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