Windjammer Partisan Voting Index for US State Houses

[muon2]

Well , I’m happy to finally introduce the  Windjammer Partisan Voting Index for US State Houses. Like the Windjammer Partisan Voting Index for US State Senates, a D+1 district means that if the presidential  democrat candidate gets 50%, in this house district, he would get 51%.

For those who like mathematics:
PVI in the House District n:  [(Obama2012 score- Romney2012 score)/2 -1.95 +(Obama2008score-Mccain2008score)/2-3.6]/2

Here we go!

Great work, but the math is a little off. You didn't divide to get the two party vote, which makes the gap smaller than it would be with the correct weighting. Using differences, the formula is

PVI(D+) = 50%*[(D2012-R2012)/2*(D2012+R2012) - 0.0197 + (D2008-R2008)/2*(D2008+R2008) - 0.0369]

[muon2]

Well , I’m happy to finally introduce the  Windjammer Partisan Voting Index for US State Houses. Like the Windjammer Partisan Voting Index for US State Senates, a D+1 district means that if the presidential  democrat candidate gets 50%, in this house district, he would get 51%.

For those who like mathematics:
PVI in the House District n:  [(Obama2012 score- Romney2012 score)/2 -1.95 +(Obama2008score-Mccain2008score)/2-3.6]/2

Here we go!

Great work, but the math is a little off. You didn't divide to get the two party vote, which makes the gap smaller than it would be with the correct weighting. Using differences, the formula is

PVI(D+) = 50%*[(D2012-R2012)/2*(D2012+R2012) - 0.0197 + (D2008-R2008)/2*(D2008+R2008) - 0.0369]

Thanks for pointing out.
Well, I understand the formula. I guess the problem with my formula is that it doesn't take into account third parties? Does that change a lot of things in the end considering third parties in the US<3%?

It would typically change the results by 0.1 or 0.2. BTW I corrected my formula, since I forgot the extra factor of two due to the fact you used the difference between the two major candidates instead of the share of the two-party vote.

However, I was looking at your IL results, and some districts seemed a bit off so I dug into the data for one district to check. I have all the PVIs for the new 2012 districts calculated with the 04-08 numbers, which were derived when the new maps were announced. Yours were a bit different, so I thought I should crosscheck one district with 08-12 numbers to confirm the discrepancy.

You list IL HD49 as PVI -1, and my 2004-2008 PVI of that district with the new boundaries was PVI -3.1. The 2012 result in that district was Obama 22.0K - Romney 23.5K or a difference of -3.3% which if I divide by 2 and subtract 1.97% gives a 2012 factor of D-3.6. Using the 2012 precincts for the new district (which has almost nothing in common with the old HD49) the 2008 vote in HD49 was Obama 23.7K - McCain 20.0K or a difference of +8.5% which gives a 2008 factor of D+0.5. When I average these two I got a net PVI(D)-1.6. It's close to yours, but it would round off to D-2.

Is it just that you have a number of HDs that are slightly more Dem than mine and rounding shifts them consistently up a point or so? If you could share your details for that HD I can confirm my suspicion.

[muon2]

Thanks, my suspicion was correct. The vote totals on your sheet are slightly different than on mine. So in the aforementioned HD49, their totals give a PVI of -1.25 and I had -1.55. As I surmised we rounded in different directions. Since they only have the two party totals the other correction I mentioned would not come into play.

In IL many precincts are split between different districts, but there is only one total for the presidential vote in that precinct. Votes from those precincts have to to split between districts based on some estimate division. My guess is that their split was different than mine.

[muon2]

Obama's 2008 performance pushed most suburban Chicago districts about 3 points more Dem in the PVI. The raw annual numbers for PVI (my values) in 04-12 were R+6.7, D+0.5, and R+3.6. The average of the first two is R+3.1 and for the second two is R+1.6. If one skips over '08 the PVI would be R+5.2. In off years the lack of D turnout runs the number in excess of R+10. 2016 will be very interesting in IL without Obama at the top of the ticket.