Margin Swings & Trends

Swing Map
Trend Map

Upon a suggestion from Alcon, I have created swing maps by county between the 2000 and 2004 elections. These maps describe the change in margin percentage (i.e. the difference between the Democratic vote percentage and the Republican vote percentage) between the 2000 election and the 2004 election. The maps may be found on the state summary pages (example). These maps show the change in support on a percentage basis for each county between the successive elections. In the New Jersey map to the right, the margin in every county swung towards Bush. The very light blue represents a change of < 5%, the light blue between 5% and 10%, the blue between 10% and 15%, and the dark blue 15% to 20% (see the Atlas Master Key for complete color and margin assignments).

(edit: 20051101) A second set of maps have been produced that show the county “trend” between the 2000 and 2004 elections. The trend map differs from the swing map in that the margin change is normalized to the nation-wide margin change (i.e. MarginCounty2004 – MarginNational2004 – (MarginCounty2000 – MarginNational2000) where each Margin is (VoteD – VoteR)/TotalVote). So, as an example, consider Hunterdon County, New Jersey:

Year VoteD VoteR TotalVote Margin Margin%
2004 26,050 39,888 66,680 13,838 20.75%
2000 21,387 32,210 56,455 10,823 19.17%
Swing 4,663 7,678 10,225 3,015 1.58%
2004 59,028,439 62,040,610 122,300,762 3,012,171 2.46%
2000 51,003,926 50,460,110 105,417,258 543,816 0.52%
Swing 8,024,516 11,580,500 16,883,504 3,555,987 2.98%
County Trend 2000 to 2004
(County Swing – National Swing) 1.40%

To calculate the swing in Hunterdon County, subtract the margin percentage in 2000 from the margin percentage in 2004 -> an increase for the Republicans of 1.58%. Therefore the swing map shows Hunterdon County for Republican by < 5%. The trend is a normalization of the county margin change based on the national margin change. The trend is calculated by subracting the national margin swing from the county margin swing. Since the margin for Republican nationwide increased by 2.98% and the margin in Hunterdon County swung a smaller 1.58% towards the Republicans, the trend in Hunterdon County is 1.40% towards the Democrats. The map therefore shows Hunterdon County for Democratic by < 5%.

Feedback is welcome on all aspects of the feature (color selection, number of bins, whether it is useful or interesting, etc.).

2 thoughts on “Margin Swings & Trends

  1. Brian

    Could you please explain the difference between the swing and trend maps? For example, how could Dade County FL swing in favor of Bush but trend Democratic? Thanks.

  2. Alcon


    Swing is the difference between the 2000 and the 2004 margin.

    Trend is the difference between the 2000 and 2004 margin when both elections are adjusted as if they were ties. In other words, 2004 is adjusted toward the Democrats by 2.47 and 2000 to the Republicans by 0.51.

    So, a swing of 0.00% is equal to a trend of 2.98%, because that is what it would have been if the elections were tied.

    Hope that clears things up!

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