andrew_gelman_stats andrew_gelman_stats-2010 andrew_gelman_stats-2010-43 knowledge-graph by maker-knowledge-mining
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Introduction: I’ll defer to Nate on the details but just wanted to add a couple of general thoughts. My quick answer is that you can’t learn much from primary elections. They can be important in their effects–both directly on the composition of Congress and indirectly in how they can affect behavior of congressmembers who might be scared of being challenged in future primaries–but I don’t see them as very informative indicators of the general election vote. Primaries are inherently unpredictable and are generally decided by completely different factors, and from completely different electorates, than those that decide general elections. The PA special election is a bit different since it’s a Dem vs. a Rep, but it’s also an n of 1, and it’s an election now rather than in November. Nate makes a convincing case that it’s evidence in favor of the Democrats, even if not by much.
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1 I’ll defer to Nate on the details but just wanted to add a couple of general thoughts. [sent-1, score-0.654]
2 My quick answer is that you can’t learn much from primary elections. [sent-2, score-0.409]
3 They can be important in their effects–both directly on the composition of Congress and indirectly in how they can affect behavior of congressmembers who might be scared of being challenged in future primaries–but I don’t see them as very informative indicators of the general election vote. [sent-3, score-1.958]
4 Primaries are inherently unpredictable and are generally decided by completely different factors, and from completely different electorates, than those that decide general elections. [sent-4, score-1.321]
5 The PA special election is a bit different since it’s a Dem vs. [sent-5, score-0.606]
6 a Rep, but it’s also an n of 1, and it’s an election now rather than in November. [sent-6, score-0.282]
7 Nate makes a convincing case that it’s evidence in favor of the Democrats, even if not by much. [sent-7, score-0.391]
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same-blog 1 1.0000001 43 andrew gelman stats-2010-05-19-What do Tuesday’s elections tell us about November?
Introduction: I’ll defer to Nate on the details but just wanted to add a couple of general thoughts. My quick answer is that you can’t learn much from primary elections. They can be important in their effects–both directly on the composition of Congress and indirectly in how they can affect behavior of congressmembers who might be scared of being challenged in future primaries–but I don’t see them as very informative indicators of the general election vote. Primaries are inherently unpredictable and are generally decided by completely different factors, and from completely different electorates, than those that decide general elections. The PA special election is a bit different since it’s a Dem vs. a Rep, but it’s also an n of 1, and it’s an election now rather than in November. Nate makes a convincing case that it’s evidence in favor of the Democrats, even if not by much.
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Introduction: At the sister blog, Henry writes about poll averaging and election forecasts. Henry writes that “These models need to crunch lots of polls, at the state and national level, if they’re going to provide good predictions.” Actually, you can get reasonable predictions from national-level forecasting models plus previous state-level election results, then when the election comes closer you can use national and state polls as needed. See my paper with Kari Lock, Bayesian combination of state polls and election forecasts . (That said, the method in that paper is fairly complicated, much more so than simply taking weighted averages of state polls, if such abundant data happen to be available. And I’m sure our approach would need to be altered if it were used for real-time forecasts.) Having a steady supply of polls of varying quality from various sources allows poll aggregators to produce news every day (in the sense of pushing their estimates around) but it doesn’t help much with a
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Introduction: Jeff the Productivity Sapper points me to this insulting open letter to Nate Silver written by pollster John Zogby. I’ll go through bits of Zogby’s note line by line. (Conflict of interest warning: I have collaborated with Nate and I blog on his site). Zogby writes: Here is some advice from someone [Zogby] who has been where you [Silver] are today. Sorry, John. (I can call you that, right? Since you’re calling Nate “Nate”?). Yes, you were once the hot pollster. But, no, you were never where Nate is today. Don’t kid yourself. Zogby writes: You [Nate] are hot right now – using an aggregate of other people’s work, you got 49 of 50 states right in 2008. Yes, Nate used other people’s work. That’s what’s called “making use of available data.” Or, to use a more technical term employed in statistics, it’s called “not being an idiot.” Only in the wacky world of polling are you supposed to draw inferences about the U.S.A. using only a single survey organization. I do
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Introduction: I’ve written a lot on polls and elections (“a poll is a snapshot, not a forecast,” etc., or see here for a more technical paper with Kari Lock) but had a few things to add in light of Sam Wang’s recent efforts . As a biologist with a physics degree, Wang brings an outsider’s perspective to political forecasting, which can be a good thing. (I’m a bit of an outsider to political science myself, as is my sometime collaborator Nate Silver, who’s done a lot of good work in the past few years.) But there are two places where Wang misses the point, I think. He refers to his method as a “transparent, low-assumption calculation” and compares it favorably to “fancy modeling” and “assumption-laden models.” Assumptions are a bad thing, right? Well, no, I don’t think so. Bad assumptions are a bad thing. Good assumptions are just fine. Similarly for fancy modeling. I don’t see why a model should get credit for not including a factor that might be important. Let me clarify. I
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Introduction: Nate Silver and Justin Wolfers are having a friendly blog-dispute about momentum in political polling. Nate and Justin each make good points but are also missing parts of the picture. These questions relate to my own research so I thought I’d discuss them here. There ain’t no mo’ Nate led off the discussion by writing that pundits are always talking about “momentum” in the polls: Turn on the news or read through much of the analysis put out by some of our friends, and you’re likely to hear a lot of talk about “momentum”: the term is used about 60 times per day by major media outlets in conjunction with articles about polling. When people say a particular candidate has momentum, what they are implying is that present trends are likely to perpetuate themselves into the future. Say, for instance, that a candidate trailed by 10 points in a poll three weeks ago — and now a new poll comes out showing the candidate down by just 5 points. It will frequently be said that this
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same-blog 1 0.95730805 43 andrew gelman stats-2010-05-19-What do Tuesday’s elections tell us about November?
Introduction: I’ll defer to Nate on the details but just wanted to add a couple of general thoughts. My quick answer is that you can’t learn much from primary elections. They can be important in their effects–both directly on the composition of Congress and indirectly in how they can affect behavior of congressmembers who might be scared of being challenged in future primaries–but I don’t see them as very informative indicators of the general election vote. Primaries are inherently unpredictable and are generally decided by completely different factors, and from completely different electorates, than those that decide general elections. The PA special election is a bit different since it’s a Dem vs. a Rep, but it’s also an n of 1, and it’s an election now rather than in November. Nate makes a convincing case that it’s evidence in favor of the Democrats, even if not by much.
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Introduction: Yesterday at the sister blog , Nate Silver forecast that the Republicans have a two-thirds chance of regaining the House of Representatives in the upcoming election, with an expected gain of 45 House seats. Last month, Bafumi, Erikson, and Wlezien released their forecast that gives the Republicans an 80% chance of takeover and an expected gain of 50 seats. As all the above writers emphasize, these forecasts are full of uncertainty, so I treat the two predictions–a 45-seat swing or a 50-seat swing–as essentially identical at the national level. And, as regular readers know, as far back as a year ago , the generic Congressional ballot (those questions of the form, “Which party do you plan to vote for in November?”) was also pointing to big Republican gains. As Bafumi et al. point out, early generic polls are strongly predictive of the election outcome, but they need to be interpreted carefully. The polls move in a generally predictable manner during the year leading up to an
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Introduction: Sandy Gordon sends along this fun little paper forecasting the 2010 midterm election using expert predictions (the Cook and Rothenberg Political Reports). Gordon’s gimmick is that he uses past performance to calibrate the reports’ judgments based on “solid,” “likely,” “leaning,” and “toss-up” categories, and then he uses the calibrated versions of the current predictions to make his forecast. As I wrote a few weeks ago in response to Nate’s forecasts, I think the right way to go, if you really want to forecast the election outcome, is to use national information to predict the national swing and then do regional, state, and district-level adjustments using whatever local information is available. I don’t see the point of using only the expert forecasts and no other data. Still, Gordon is bringing new information (his calibrations) to the table, so I wanted to share it with you. Ultimately I like the throw-in-everything approach that Nate uses (although I think Nate’s descr
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Introduction: Political campaigns are commonly understood as random walks, during which, at any point in time, the level of support for any party or candidate is equally likely to go up or down. Each shift in the polls is then interpreted as the result of some combination of news and campaign strategies. A completely different story of campaigns is the mean reversion model in which the elections are determined by fundamental factors of the economy and partisanship; the role of the campaign is to give voters a chance to reach their predetermined positions. The popularity of the random walk model for polls may be partially explained via analogy to the widespread idea that stock prices reflect all available information, as popularized in Burton Malkiel’s book, A Random Walk Down Wall Street. Once the idea has sunk in that short-term changes in the stock market are inherently unpredictable, it is natural for journalists to think the same of polls. For example, political analyst Nate Silver wrote
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Introduction: Nate Silver and Justin Wolfers are having a friendly blog-dispute about momentum in political polling. Nate and Justin each make good points but are also missing parts of the picture. These questions relate to my own research so I thought I’d discuss them here. There ain’t no mo’ Nate led off the discussion by writing that pundits are always talking about “momentum” in the polls: Turn on the news or read through much of the analysis put out by some of our friends, and you’re likely to hear a lot of talk about “momentum”: the term is used about 60 times per day by major media outlets in conjunction with articles about polling. When people say a particular candidate has momentum, what they are implying is that present trends are likely to perpetuate themselves into the future. Say, for instance, that a candidate trailed by 10 points in a poll three weeks ago — and now a new poll comes out showing the candidate down by just 5 points. It will frequently be said that this
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same-blog 1 0.95002764 43 andrew gelman stats-2010-05-19-What do Tuesday’s elections tell us about November?
Introduction: I’ll defer to Nate on the details but just wanted to add a couple of general thoughts. My quick answer is that you can’t learn much from primary elections. They can be important in their effects–both directly on the composition of Congress and indirectly in how they can affect behavior of congressmembers who might be scared of being challenged in future primaries–but I don’t see them as very informative indicators of the general election vote. Primaries are inherently unpredictable and are generally decided by completely different factors, and from completely different electorates, than those that decide general elections. The PA special election is a bit different since it’s a Dem vs. a Rep, but it’s also an n of 1, and it’s an election now rather than in November. Nate makes a convincing case that it’s evidence in favor of the Democrats, even if not by much.
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Introduction: Upon reading this note by John Cook on continued fractions, I wrote: If you like continued fractions, I recommend you read the relevant parts of the classic Numerical Methods That Work. The details are probably obsolete but it’s fun reading (at least, if you think that sort of thing is fun to read). I then looked up Acton in Wikipedia and was surprised to see he’s still alive. And he wrote a second book (published at the age of 77!). I wonder if it’s any good. It’s sobering to read Numerical Methods That Work: it’s so wonderful and so readable, yet in this modern era there’s really not much reason to read it. Perhaps William Goldman (hey, I checked: he’s still alive too!) or some equivalent could prepare a 50-page “good parts” version that could be still be useful as a basic textbook.
3 0.88233775 579 andrew gelman stats-2011-02-18-What is this, a statistics class or a dentist’s office??
Introduction: Val has reported success with the following trick: Get to the classroom a few minutes earlier and turn on soft music. Then set everything up and, the moment it’s time for class to begin, put a clicker question on the screen and turn off the music. The students quiet down and get to work right away. I’ve never liked the usual struggle with students to get them to settle down in class, as it seemed to set up a dynamic in which I was trying to get the students to focus and they were trying to goof off. Turning off the music seems like a great non-confrontational way to send the signal that class is starting.
4 0.8821643 646 andrew gelman stats-2011-04-04-Graphical insights into the safety of cycling.
Introduction: This article by Thomas Crag, at Copenhagenize, is marred by reliance on old data, but it’s so full of informative graphical displays — most of them not made by the author, I think — that it’s hard to pick just one. But here ya go. This figure shows fatalities (among cyclists) versus distance cycled, with a point for each year…unfortunately ending in way back in 1998, but still: This is a good alternative to the more common choice for this sort of plot, which would be overlaying curves of fatalities vs time and distance cycled vs time. The article also explicitly discusses the fact, previously discussed on this blog , that it’s misleading, to the point of being wrong in most contexts, to compare the safety of walking vs cycling vs driving by looking at the casualty or fatality rate per kilometer . Often, as in this article, the question of interest is something like, if more people switched from driving to cycling, how many more or fewer people would die? Obviously, if peo
Introduction: Tyler Cowen hypothesizes a “dogmatism portfolio” or a “quota of dogmatism”: in his words, If you’re very dogmatic in one area, you may be less dogmatic in others. OK, well “may be” is pretty vague. There’s not really anything to disagree with, yet. But then Cowen continues: There’s a lesson here. If you wish to be a more open-minded thinker, adhere to some extreme and perhaps unreasonable fandoms, the more firmly believed the better and the more obscure the area the better. This will help fulfill your dogmatism quota, yet without much skewing your more important beliefs. He seems to be making a testable prediction here, that levels of dogmatism on two randomly chosen issues should be negatively correlated. I guess I should call this “almost testable,” as it still requires an issue-by-issue measure of dogmatism. (Is it dogmatic to believe that there was this guy called Jesus who walked on water . . . or is it dogmatic to say that Jesus didn’t walk on water and
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