hPVI 2017 Explainer
Rejoice! The thing every Minnesota politico has been waiting for all year, the release of hPVI, is finally here and just in time for it to still be called the 2017 edition. Let me know when and where the parade will be held.
This post right here though doesn’t actually have any of the hPVIs, this post right here is the explainer post. For those of you wondering what hPVI is, this is the post for you.
For those of you who want the actual goodness (read: the numbers), those will come out in separate posts.
The hPVI Explainer
“What is hPVI?”
hPVI is a Partisan Voting Index designed for Minnesota legislative districts (but could be used for any state really). hPVI is based on Cook PVI, from Wikipedia:
The Cook Partisan Voting Index (Cook PVI) is a measurement of how strongly a United States congressional district leans toward the Democratic or Republican Party, compared to the nation as a whole. The Cook Political Report introduced the PVI in August 1997 to better gauge the competitiveness of each district using the 1992 and 1996 presidential elections as a baseline
PVI uses the results of the two most recent Presidential elections to compute a partisan index for a congressional district. The partisan index can then be used to compare districts to one another and predict how they will vote in future congressional elections.
“Why not just look at the results of the congressional elections themselves?”
Simply, every congressional election is different. The candidates are different, the issues are different, the context is different. On the other hand, everyone in every congressional district has the same two choices for President (PVI and hPVI is a two-party based metric, it pretends third parties don’t exist). Since the point of PVI is to compute a baseline partisan index for a district, it’s better to use the numbers from the candidates who ran in all of the congressional districts for this purpose.
“Okay, now that you’ve so ably explained PVI, what is hPVI?”
Hybrid Partisan Voting Index. Hybrid because while it is a very similar metric to PVI, it uses slightly different data and is calculated differently.
Before explaining how hPVI is calculated, here’s Wikipedia again explaining how PVI is calculated:
PVIs are calculated by comparing the district’s average Democratic or Republican Party’s share of the two-party presidential vote in the past two presidential elections to the nation’s average share of the same. The national average for 2004 and 2008 was 51.2% Democratic to 48.8% Republican. For example, in Alaska’s at-large congressional district, the Republican candidate won 63% and 61% of the two-party share in the 2004 and 2008 presidential elections, respectively. Comparing the average of these two results (62%) against the average national share (49%), this district has voted 13 percentage points more Republican than the country as a whole, or R+13.
PVI uses the election results from the past two Presidential elections (in this case it would be 2016 and 2012) while hPVI instead uses the results from the last Presidential election (2016) and the last Gubernatorial election (2014). This change has two advantages; newer data, and a set of local data. In any given iteration of hPVI, the oldest data will be from four years prior while with PVI the Presidential data can be as old as eight years. Additionally, having a set of data (in this case from the last Governors race) that is local to the state helps to smooth over any disconnects between voters perceptions of the state versus the national parties.
The other thing that differs between the two metrics is that PVI is calculated as the difference between how the district voted and how the country as a whole voted. hPVI used to be calculated that way only instead of comparing how a district had voted to the whole country, it compared how it voted to the state. This was changed in 2013 to make hPVI more accurate in predicting the results of Minnesota legislative races. Now hPVI is calculated as the difference between how the district voted and 50%.
For programatic access to the numbers see the hPVI API. For more information about how the numbers were computed see the GitHub repo.
“Alright, what do the numbers mean though? How good is an hPVI of 3 or 9?”
Using the toss-up, lean, likely, safe scale of district rating: An hPVI of greater than about 10 would be safe. An hPVI of about 6 or greater would be a likely. An hPVI of about 3 or greater would be a lean. Everything else would be a toss-up.
This isn’t to say that every district with an hPVI of 12 will be safe, or every district with an hPVI of 3 is a leaner. The actual candidates and context of those races matter as well and can shift a rating in one direction or another.
To answer the fake question I posed to myself, an hPVI of 3 is a competitive district, while a district with an hPVI of 9 is not typically a competitive district, unless of course Matt Little is running.
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