Despite what I would label as lukewarm coverage for Mitt Romney during the RNC, the convention and its four days of media coverage (hurricane and all) produced a very large and positive shift in political Win Probability (pWP) for the Republican Nominee.
Here is a graph showing Obama’s level of support, and his calculated pWP, per each poll I recorded:
I calculate everything in terms of Obama’s pWP and level of support, as he is the incumbent. Whenever I do calculations for other races, I’ll maintain the same convention.
In order to get a clearer picture of how events effect pWP, we have to aggregate the data, post hoc, based on what is, in my opinion, the primary driver of news coverage over the periods in question. Here is the graph of aggregate pWP for Obama:
Some notes on the aggregated data:
- The RNC was worth a solid 34.6 pWPA (political Win Probability Added). By this I mean, based strictly on the polls, Romney’s chance of winning the election increased from just under 30% to almost 70%. It more than doubled.
- Obviously, the DNC, if traditional wisdom holds true, should help Obama in the same way. I think the conventions are low leverage events, so their effect should be fleeting. We’ll get a good chance to see if I’m right in the coming weeks. It will also be interesting to see if Obama can recapture all of his pre-RNC pWP.
- Initial polling in the summer, after Romney had secured the nomination and before he picked Paul Ryan, Obama’s pWP was around 60%. Since FDR, presidential incumbents win 62% of the time, or 60% (depending on whether one counts presidents who previously won an election, or succeeded to office after the death or resignation of the previous president); of all the presidents seeking reelection since Washington, 58% of them win. That’s a nice convergence of current data and historical data.
- The group of polls around the time after Paul Ryan was picked as the VP candidate showed a positive movement for Romney. Then, after a while, it looks like a strong TV ad campaign starting around the time of the Olympics helped Obama, leading to some very high pWP numbers just before the RNC.
- I do not weight the polls based on sample size when aggregating. My reasoning is simple, I do not know a bad sample from a good sample, I don’t want to weight a poorly constructed poll with a large sample size more than a poll with a small sample size that is properly constructed. My assumption is the sampling errors will cancel themselves out as more polls are added. (I also do not discriminate polls, I use all Likely Voter polls, regardless of bias of the polling institution, with the assumption the errors, or bias, will cancel out.)
- This post is long enough, I’ll talk about some of my methods, and the strengths and weaknesses thereof, in a later post.