Introducing the 2009 update to the North Carolina Partisan Index (NCPI), a new way to identify districts that swing, lean, or firmly trend towards one political party or the other.
In early 2008, the Civitas Institute premiered the North Carolina Partisan Index using data from the 2004 General Election. This year, we have updated the NCPI to reflect voters’ choices in the 2008 General Election.
Modeled after the Cook Partisan Voting Index developed for congressional districts, the North Carolina Partisan Index compares the political leanings of voters in each state house and senate district with the partisan voting tendencies of the state as a whole. The end result is a letter (D or R) followed by a number, indicating the extent to which each district leans one way or the other.
The new NCPI was developed using adjusted 2008 data on the elections for Governor and other council of state offices – Lieutenant Governor, Attorney General, Commissioners of Agriculture, Labor, and Insurance, Secretary of State, State Auditor, State Treasurer, and Superintendent of Public Instruction.
|2008 House Detail||2008 Senate Detail|
|2009 House Update|
The 2008 NCPI map for State House Districts has been updated to reflect the changes in the House Redistricting Plan modified by House Bill 1621. The changes are limited to House districts 16 and 18 in Pender and New Hanover counties.
This redistricting did little to impact the NCPI data, only House District 16 showed a decrease from R+9 to R+8 while House District 18 remained at D+12.
Changes This Year
Compared to the 2004 NCPI model, the 2008 NCPI shows a greater concentration of Democratic-leaning voters in the urban population centers. While voters in most of the state were somewhat more likely to vote for Republican statewide candidates in 2008 compared to 2004, voters in much of Buncombe, Cumberland, Durham, Forsyth, Guilford, Mecklenburg, and Wake counties favored Democratic candidates more heavily. In fact, 19 of the 21 state House districts that voted significantly more Democratic in 2008 compared to 2004 ‚Äì districts whose NCPI moved more than 3 points Democratic ‚Äì are in those seven most populous North Carolina counties.
How Does It Work?
The NCPI compares votes cast in each district to votes cast in the state as a whole, allowing us to see the propensity, or political leanings, of voters in that district. The NCPI does not by itself predict election outcomes. Rather, it is a valuable tool in determining the relative likelihood of voters to elect a Democratic versus Republican state legislator, all other things being equal.
As an example, a district whose voters allotted 5 more percentage points to the Democratic candidates compared to the state average receives an index score of D+5. A district whose voters allotted 5 more percentage points to the Republican candidates receives a score of R+5.
How Well Does It Work?
In 2008, the outcome in state House races was more highly correlated with the NCPI than with voter registration. In fact, holding party registration and race constant, on average a 1 percentage point increase in the NCPI (i.e. from D+1 to D+2) translated to an increase of 0.94 percentage points in the vote for that party’s candidate. Of the 60 contested legislative seats in 2008, all but 10 seats went to the winner of the party indicated by the NCPI (three seats had a neutral NCPI). Among the 10 districts that were not in line with the NCPI score, eight were in the range of R+3 to D+3, and most involved races with multi-term incumbents, well-known challengers, or significant spending differences between the candidates. When the value of incumbency is taken into account, the outcome of all but one state House race is correctly predicted by the NCPI model.
More About The Model
The NCPI is based on voter data from the 2008 general election results for governor and other council of state offices. Although president and U.S. Senate results were also available, we found that state-level races give a more accurate picture of how voters will vote in a state legislative race than do national races.
In council of state races, North Carolinians tend to vote for Democratic candidates ‚Äì the average council of state vote (looking at only votes for Democratic or Republican candidates) was 53.4 percent Democratic and 46.6 percent Republican. However, many council of state seats have been unevenly contested in the past, making it difficult to predict results “down ticket‚” looking at raw numbers alone. By looking only at deviations from the state average, the Civitas model is able to mitigate the effects of incumbency and uneven contests.
The NCPI model also adjusts for outliers due to the popularity of a specific candidate in a specific area (for example, the governor’s popularity in her home county). Other data assumptions are also applied.