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198 brendan oconnor ai-2013-08-20-Some analysis of tweet shares and “predicting” election outcomes


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Introduction: Everyone recently seems to be talking about this newish paper by Digrazia, McKelvey, Bollen, and Rojas  ( pdf here ) that examines the correlation of Congressional candidate name mentions on Twitter against whether the candidate won the race.  One of the coauthors also wrote a Washington Post Op-Ed  about it.  I read the paper and I think it’s reasonable, but their op-ed overstates their results.  It claims: “In the 2010 data, our Twitter data predicted the winner in 404 out of 435 competitive races” But this analysis is nowhere in their paper.  Fabio Rojas has now posted errata/rebuttals  about the op-ed and described this analysis they did here.  There are several major issues off the bat: They didn’t ever predict 404/435 races; they only analyzed 406 races they call “competitive,” getting 92.5% (in-sample) accuracy, then extrapolated to all races to get the 435 number. They’re reporting about  in-sample predictions, which is really misleading to a non-scientific audi


Summary: the most important sentenses genereted by tfidf model

sentIndex sentText sentNum sentScore

1 Everyone recently seems to be talking about this newish paper by Digrazia, McKelvey, Bollen, and Rojas  ( pdf here ) that examines the correlation of Congressional candidate name mentions on Twitter against whether the candidate won the race. [sent-1, score-0.41]

2 There are several major issues off the bat: They didn’t ever predict 404/435 races; they only analyzed 406 races they call “competitive,” getting 92. [sent-6, score-0.44]

3 These aren’t predictions from just Twitter data, but a linear model that includes incumbency status and a bunch of other variables. [sent-9, score-0.578]

4 If you look at their Figure 1, as Nagler reproduces, it’s obvious that tweet share alone gives much less than that much accuracy. [sent-13, score-0.672]

5 Thus, if you say “predict the winner to be whoever got more tweet mentions,” then the number of correct predictions would be the number of dots in the shaded yellow areas, and the accuracy rate are them divided by the total number of dots. [sent-18, score-0.949]

6 [1] It’s also been pointed out that incumbency alone predicts most House races; are tweets really adding anything here? [sent-20, score-0.341]

7 The main contribution of the paper is to test tweets alongside many controlling variables, including incumbency status. [sent-21, score-0.509]

8 The most convincing analysis the authors could have done would be to add an ablation test: use the model with the tweet share variable, and a model without it, and see how different the accuracies are. [sent-22, score-0.972]

9 One additional percentage point of tweet share is worth 155 votes. [sent-27, score-0.821]

10 [2]  The predictive effect of tweet share is significant, but small. [sent-28, score-0.616]

11 In the paper they point out that a standard deviation worth of tweet share margin comes out to around 5000 votes — so roughly speaking, tweet shares are 10% as important as incumbency? [sent-29, score-1.539]

12 On the other hand, tweet share is telling something that those greyed-out, non-significant demographic variables aren’t, so something interesting might be happening. [sent-32, score-0.699]

13 The paper also has some analysis of the outliers where the model fails. [sent-33, score-0.373]

14 It’s scientifically irresponsible to take the in-sample predictions and say “we predicted N number of races correctly” in the popular press. [sent-43, score-0.668]

15 ”  In-sample predictions are a pretty technical concept and I think it’s misleading to call them “predictions. [sent-46, score-0.406]

16 I feel a little bad for the coauthors given how many hostile messages I’ve seen about their paper on Twitter and various blogs; presumably this motivates what Rojas says at the end of their errata/rebuttal : The original paper is a non-peer reviewed draft. [sent-48, score-0.391]

17 [1] Also weird: many of the races have a 100% tweet share to one candidate. [sent-55, score-0.935]

18 [2] These aren’t literal vote counts, but number of votes normalized by district size; I think it might be interpretable as, expected number of votes in an average-sized city. [sent-61, score-0.571]

19 Some blog posts have complained they don’t model vote share as a percentage, but I think their normalization preprocessing actually kind of handles that, albeit in a confusing/non-transparent way. [sent-62, score-0.512]

20 5; so I guess that’s more like, a standardized unit of tweet share is worth 20% of standardized impact of incumbency? [sent-65, score-0.921]


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wordName wordTfidf (topN-words)

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