December 14, 2004
Exits: Were They Really "Wrong?"
Last week's posting of more detailed information on the sampling error of exit polls by the National Election Pool (NEP) allows for a quick review of the now well established conventional wisdom that the "exit polls were wrong."
Let's first set aside the mid-day numbers that were widely leaked on the Internet but never intended as the basis for projections (numbers that even the exit pollsters recognized as flawed in some states -- see this earlier post for explanations on the differences among the many estimates provided by the exit pollsters on Election Day). Let us also stipulate that at least one state estimate of Hispanic voters (Texas) was obviously wrong, given the correction issued by NEP.
The conclusion that the exit polls were wrong is essentially about two possible failings:
- That the end-of-day numbers seemed to predict a Kerry victory.
- That the end-of-day numbers showed Kerry doing consistently better than he actually did.
What is the reality given what we now know about the exit poll's sampling error?
1) Did the just-before-poll-closing exit polls show a consistent and statistically significant "error" in Kerry's favor?
Yes, but that error has been exaggerated. Here is what we know:
- An internal NEP review of 1,400 sample precincts showed Kerry's share of the vote overstated by an average of 1.9 percentage points. As far as I can tell, no one from NEP questions the statistical significance of that overstatement.
- The before-poll-closing exit poll results posted by Steven Freeman (and included in the updated report by the Cal Tech / MIT Vote Project) show errors in Kerry's favor in 43 of the 51 surveys (the states plus DC). These overstate Kerry's vote by an average of 1.6 percentage points. If the surveys had been perfectly executed with perfectly random samples (an all but impossible assumption under real world conditions), the pattern of errors should have been the same as if we had flipped a coin 51 times: about half favoring Kerry, about half favoring Bush. The probability of 43 of 51 perfect surveys favoring Kerry by chance alone is less than 0.001%. Of course, this statistic only tells us that the surveys were imperfect. It says nothing about the cause or magnitude of the error.
To be clear: Everyone -- including the exit pollsters -- agrees they "overstated" Kerry's vote. There is some argument about the precise degree of certainty of that overstatement, but if all agree that the difference is statistically significant the degree of certainty has little consequence. The size of the error matters, and the reasons for it matter, but whether our level of confidence about the error's existence is 99.9% or something greater does not.
Having said that, the second draft of the paper, "The Unexplained Exit Poll Discrepancy," continues to needlessly exaggerate the significance of the error, especially within individual states. For example, Freeman claims that there were significant "discrepancies" in Pennsylvania, Ohio and Florida when each state is considered separately. He has a graphic (Figure 1.2) showing a significant error in Florida, assuming a 95% confidence level. However, these assertions are not supported by the margins of error reported by NEP.
- I applied the appropriate "confidence intervals" reported by NEP (as distributed to its partner networks on or before Election Day) to each state. Contrary to Freeman's assertions, the separate "discrepancies" in Ohio, Florida and Pennsylvania fail to attain statistical significance even at a 95% confidence level. In fact, I see only four states (New Hampshire, New York, South Carolina and Vermont) with statistically significant errors favoring Kerry at a 95% confidence level. Of course, when using a 95% confidence level, 2-3 states should be out of range by chance alone.
- I also followed the advice of Nick Panagakis and estimated confidence intervals at a 99.5% level of confidence (the standard used by NEP to make projections) using the actual estimates of the design effect obtained from
NEPWarren Mitofsky by blogger Rick Brady. By my calculations, this more demanding test renders the apparent errors in NH, NY, SC and VT non-significant.
Then there is the statistic heard round the world - Freeman's original "250 million to one" estimate of the odds against the discrepancy occurring simultaneously in Pennsylvania, Ohio and Florida. In his second draft, Freeman revised his estimate down to a mere 662,000 to one. Given that Freeman continues to understate the "design effect" used to calculate the sampling error in this year's exit polls, his revised estimate also remains too high.
Some have asked that I calculate my own estimate of the joint probability of an error in Ohio, Florida and Pennsylvania. I am reluctant to do so for two reasons: First, the rounding error in Freeman's data alone renders this sort of hairsplitting moot. Second, and more important, it really doesn't matter. Everyone concedes there was a small (2%) but significant average error in Kerry's direction. For those concerned about problems with the count, what matters most is why that error occurred.
2) Considering sampling error, did the end-of-day numbers predict a Kerry victory?
This one is easy: No. Not even close.
It is true that the early numbers fooled a lot of people, including the talking heads on the cable networks, pollsters and advisors of both campaigns and possibly -- depending on which accounts you believe -- even John Kerry and George Bush. The reason, as the Post's Richard Morin put it, is that the 1.9% average error in Kerry's favor "was just enough to create an entirely wrong impression about the direction of the race in a number of key states and nationally."
True. But that error was not big enough to give Kerry statistically significant leads in enough states to indicate a Kerry victory. If we totally ignore sampling error, the exit polls showed Kerry ahead in only four states that he ultimately lost: Ohio, Iowa, Nevada and New Mexico. Obviously, Kerry would have won the election had he prevailed in all four.
Before considering whether any of those leads were statistically significant, two quick reminders: First, as Nick Panagakis points out, NEP required at least a 99% confidence level for projections on Election Night. Second, the margin between two candidates on a survey involves two estimates; and the margin of error applies separately to each candidate. Statisticians debate which rules of thumb to apply when determining the significance of the margin between two candidates, but the consensus in the case falls somewhere between the 1.7, as recommended by the American Statistical Association and 2.0, as recommended by those who consider it more appropriate in a race where the vote for 3rd party candidates is negligible (this is a great topic for another day's post - thanks to alert reader Bill Kaminsky for guiding MP through the competing arguments).
Fortunately, in this case, the statistical debate is irrelevant. None of Kerry's apparent exit poll leads in the four states were large enough to attain statistical significance, even if we assume a 95% confidence level and use a cautious multiplier (1.7) on the margin of error. As the preceding table shows, the exit polls had Kerry ahead by 4 percentage points in Ohio, by 3 in New Mexico, by 2 in Iowa and by one point in Nevada. The NEP 95% margin of error for these states multiplied by 1.7 is between 5 and 7 percentage points. At the more appropriate confidence level of 99.5% -- the one NEP actually uses to recommend projections - these relatively small margins would fall far short of that needed to call a winner.
In a year-end review in this week's Roll Call (subscription required), Stuart Rothenberg concluded the following:
The problem wasn't the exits - it was the folks who treated the early-afternoon numbers as if they were a predictor of what would happen after everyone had voted. The exit poll was off by a couple of points, but that's well within the margin of error.
If only someone had warned us about those early afternoon numbers on Election Day. Oh, wait...
UPDATE: I mentioned William Kaminski's blog, but not his commentary on this issue nor his elegant graphic -- it's worth the click just for the chart. My only remaining quibble with his graphic is that it displays a "safe estimate" of a 95% confidence of +/-4%; leaving the impression that the numbers for DE, AL, AK and NE fall outside that interval. However, the appropriate 95% confidence interval provided by NEP for these states is +/-5%.
Related Entries - Exit Polls
Your analysis is persuasive. The one point I've wondered about in relation to the charges of fraud is whether the largest discrepancies between exit poll and vote data occur in states where control of the voting system is both partisan and under Republican control, as seemed to be the case in Florida in 2000. I don't see much evidence of that in the states you mention, but wonder if it has been looked at systematically.
Posted by: howard schuman | Dec 14, 2004 6:45:36 PM
I'm just a lawyer but as I have told several mathematicians, this is important so I am trying very hard to learn about the holy grail of "design defect." The first thing I have learned is that some mathmeticians think this notion of "design defect" is completely bogus.
Others think it is a number pulled out of someone's butt to make exit polling match actual polls.
I initially bought the concept but now I am beginning to have second doubts.
I understand the concept of clustering. If you put 10 clusters of 50 red marbles in a jar and 10 clusters of 50 blue marbles in a jar and don't mix up the jar, obviously if you take 50 marbles out at random you run the risk of getting 30 or 35 reds or 30 or 35 blues instead of something close to 25 each. So I understand if you have clustering, you have to account for this effect.
The argument goes that you have clustering (democrats and republicans live in geographic clusters)in exit polling so you must account for this effect. But the effect of clustering only occurs with small sampling. If you sample 1000 clustered jars, the 35 reds you get in one jar will be offset by 35 blues in another and the effect at some point will cancel itself out.
The higher the population, the more samples will be required; thus as our population has increased, the design effect should be headed towards extinction. Surprisingly though, the "experts" say the design defect is heading in the other direction. According to your recent post, pollsters used to account for a 30% design defect in 1996 (I think) and now it has increased to between 50-80%.
My second concern is whether we really are in fact polling in "clustered" places. More and more I see graphs of a purple America, not a blue and red one. Yes there are a few truly red areas and a few truly blue areas, but I have a hard time believing that the average polling place is much greater than a 70/30 split. Probably most are less than that. This would certainly reduce any design defect.
Third. If there is a design defect due to the clustering of exit polling, it should be evident when compared to pre-election polling which is supposed to be random. Random sampling and clustered sampling are not supposed to produce the same results. But in a study done on Democratic Underground by a mathemetician who calls himself Truth is All he compared the very last pre-election polls with the latest "non-compromised" post election polls and instead of the numbers being different as one would expect if there were a design defect, they were almost identical. He is one of the mathematicians who thinks the design defect concept is bogus to begin with. He was not attempting to show the design defect was a bogus concept by the study. I pointed it out to him. He thinks the design defect is so bogus he doesn't even want to engage in the argument. But his results should be evidence that the claimed design effect doesn't exist or has cancelled itself out due to the volume of samples.
My fourth problem is a business one. If you include the design defect in your calculations it means that margins of error are easily 3.5% in the best of situations. I can not imagine networks paying the millions of dollars they pay for election night to only be within a 3.5% margin of error when the polls close. You would not be able to call any of the swing states on anything like the timetable the networks want. I don't buy the argument that they pay all this money for other data. Sorry.
Fifth. Another mathematician on Democratic Underground, (who believes in the concept of clustering) has reworked the numbers using the highest numbers for the design defect you have given. His opinion of the data is opposite yours. His name is macdonald. But he has no idea what the real numbers for design defect should be; he's just using the latest ones you have.
Sixth: Why were exit polls so accurate in the past but not now?
Seventh: Why are they still accurate in Europe. Presumably they have political clusters there and they can get it right.
Eighth: Could it be that whatever factor you use for clustering is only as good as the honesty of the last election?
Ninth: Could it be that computer fraud has caused these numbers to inflate?
Tenth: Another mathematican on Democratic Undergrond has shown that the exit polls of those states with paper ballots were far closer than those that used paperless systems. His name is jwmealy. He suspects computer fraud.
So please forgive me if some of us don't buy the cluster argument yet.
You can check these other mathemeticans out at:
I do know of one way to test the clustering phenomonon I think. Randomly poll about 40,000 Americans now and see how they voted. That should give us a pretty accurate idea about a lot of things, including the right numbers to use for the clustering effect.
Posted by: davidgmills | Dec 14, 2004 9:14:25 PM
One thing I wish you would follow up is this: I object to introducing conventional scientific practice around "significant digits" into the discussion, as some sites have done. The scientific rules for significant digits come from "error propagation" formulae, which are derived by adding an infinitesimal (p + dp) and (q+dq) into the formulae, applying binomial expansions everywhere you can and keeping only the linear term. The dp or dq is then estimated by assuming it is the sample standard deviation and that this sample SD in turn has no uncertainty. The net result is a set of rules of thumb that you will recognize is fully equivalent to a linear model of the statistics [there are special rules for propagating random errors vs. systematic bias], with all the things that might go wrong, and none of the advantages of just doing statistics the usual way.
"Significant digits" is just a scientist's version of "poor man's statistics" assuming a one-sigma confidence interval, BTW.
Therefore, arguing (here? and in the "stones-cry-out" link) that "siginficant digits" should be used in the calculations is to argue that two corrections for the same effect (you have to get your linear model calculations correct *and* do error propagation) should be applied to the same calculation, as well as mixing a set of rules of thumb for one confidence interval with calculations targeting another.
I agree that using whole percentages introduces some, supposedly unbiased, roundoff error, but why not correct for that by modelling it and estimating its contribution to the variance? Pushing significant digits and error propagation formulae in this context is better than nothing, but quite dangerous given how many assumptions go into the rules of thumb.
Now, it *is* relevant--and I haven't seen a discussion I believe yet--that the statistics of p-q which are held to be significant, should have a different distribution from p alone, or q alone. But it isn't a t-distribution is it (and certainly not "twice the standard error" or twice the variance or twice the variance! Also, because these are percentages, not raw scores, shouldn't they be stabilized by taking a logarhithm or something?
Now, I freely admit all I know about statistics I learned from Box, Hunter, and Hunter so I could pass my CQE exam, and I'm a scientist not a statistician, but it is quite surprising that a month after the election I still haven't seen a single statistical analysis a medium statistically-educated scientist would be willing to believe about the distribution of p-q and margins of error.
Can we have a general linear model and some argument about how the residuals are distributed and brackets on estimated errors ? This is getting silly....
Posted by: John Goodwin | Dec 14, 2004 10:21:06 PM
"He thinks the design defect is so bogus he doesn't even want to engage in the argument."
This is too funny. I bet you wonder why MP won't engage you in argument.
David g Mills - may I ask where you went to Law School? I don't mean to insult you, but this really is not complex stuff. Talk to a statistician who specializes in polling. Mathemeticans know math. Statisticians know stats.
Here's a lawyer analogy for ya. Would you go to a divorce lawyer to get intellectual property advice? I wouldn't - unless you want to get hosed.
Posted by: Rick Brady | Dec 15, 2004 12:31:34 AM
John Goodwin, you are obviously smarter than all of us, this is all we are saying...
The data Freeman relies upon has been extrapolated from Male and Female proportions for Bush and Kerry rounded to the 100th. Freeman's extrapolation yielded Bush%=.479/Kerry%=.521. He used this in his calculations. However, the 1000th is not certain, it could actually be anywhere between .475 and .484 for Bush and .515 and .524 for Kerry. Are you saying there is a mathematical way to determine where the "likely" 1000th is in this range? If you are, please explain in dumb person terms how to do this and I'll take it to my stats prof. He says its impossible.
Another look at Ohio:
When the Normdist function is run for a two-tail Z-test (aka t-test) using the upper and lower bound proportions rounded to the 1000th (using the Mitofsky methods), the Z-scores (magnitude of discrepancy) and p-values (probability the magnitude can be explained by random sampling error) are as follows:
[Range] (Z-scores) [p-values]
[Bush] (-1.44 to -1.94) [.052 to .150]
[Kerry] (1.39 to 1.88) [.060 to .166]
If you use proportions rounded to the 100th, you get:
[-1.66] (0.096) [1.66] (0.096)
Now take the extrapolation from the Freeman data (to the 1000th):
[-1.72] (0.086) [1.72] (0.086)
This leads us "non-cool" scientists to believe that the data are too fuzzy for more refined analysis. I'm arguing (and my stats professor agrees) that given the data available, the "range" approach is the best method to *estimate* the significance of the dispcrepancy.
This is all academic anyhow. Where the discrepancy is "significant" (in the statistical sense) is not the issue. As MP has posted, even the pollsters say a portion of the discrepancy cannot be explained by sampling error.
If you would like to work with me to help nail down a method that you would find suitable, I'd be more than willing. I'm learning here, but so far the many "experts" I have consulted seem to be saying MP and I are right about the significant digits issue. That doesn't mean that we are right, only that no one has yet to explain how this issue DOESN'T mean the Z-score and p-values fall randomly within a "range" of possibilites.
Posted by: Rick Brady | Dec 15, 2004 1:04:46 AM
First you called me a conspiracy theorist. I'm sorry but that is the langurage of politicians and media whores.
Now you tell me that I am foolish for asking questions that highly qualified scientists outside the discipline of statisitics are asking.
I have been a lawyer for 27 years and I have been in a highly specialized field the whole time. If I need to get advice from another specialized lawyer, I certainly do. But I wouldn't dis another lawyer who is not a specialist in my field, just because he is not in my field. Sometimes a different point of view is insightful.
What I object to about the holy grail of "design defect" is that there is no proof that it actually exists. I can see how it theoretically exists, but where is the proof that it actually exists?
The phenomonon of design defect could be attributed to something totally unrealted to clustering. In fact, since design defect seems to be going up, I bet there is a direct historical correlation between the systematic use of computerized voting and the need to increase the design defect numbers. There may be more of a correlation there than there is with geographic clustering.
Just because there is a history of exit polls not matching actual polls doesn't mean you can artifically attribute this to design defect. When you have seen as much fraud as I have seen in business and politics, my first thought when anomolies pop up is to look for fraud, not some complicated, sophisticated reason to explain anomolies. I'm sure you have heard of the medical adage, "When you hear hoof beats, think horses, not zebras."
When there are huge amounts of money and huge amounts of power at stake and domething looks fishy, rule out fraud first, before you look for a highly sophisticated, complicated reason to explain the anomoly. I don't think that was the mentality of the statisticians that came up with design defect theory. I also don't think it was in their financial interest.
Posted by: davidgmills | Dec 15, 2004 8:28:15 AM
First, to Mark: Thanks for another thought-provoking commentary.
To David Mills:
It's called a design effect, not a design defect. Design effects are a well understood part of survey sampling theory and practice, and have been used successfully for decades. But you need the raw data to work them out. Analysts like Freeman have had to guess what the design effects might be (e.g. based on similar previous studies) until NEP calculated and published them.
To Rick Brady:
I think your range approach should give a good first approximation. However it assumes that Z-scores and p-values are linear functions of the proportions, which is not true, so I don't think it's necessarily the best approach. Another approach people have used with coarsened data like this is to put a uniform prior distribution on the range of possible values, and calculate intervals using Bayesian machinery (or some Monte Carlo approximation like multiple imputation). But I suspect this wouldn't make much difference, given the relatively small ranges for the proportions.
Posted by: James Reilly | Dec 15, 2004 9:07:15 AM
Design effect rather than defect. Probably a Fruedian slip.
One more thing. A comment on junk science.
In the legal community, before an expert is allowed to testify, he must convince the court that his opinion is not based on junk science. He must demonstrate that his opinion has been subjected to peer review and usually must demonstrate that support for his opinion has been published in respected journals.
Where is the rigorous academic peer review on this subject and where are the publications in highly respected journals? Where is the academic consensus on the actual number or value (in percent or otherwise) as to what this effect might be?
What raises the red flag for me is the comment that eight years ago the design effect was 30% but this year it is expected to be bewteen 50-80%. And they way these figures were published on the interenet was verbal: I heard from so and so, who heard from so and so, who got it from so and so. And these numbers are coming from people who clearly have an economic interest in the matter.
I will readily admit that the design effect phenomonon could exist. But what I will not buy without scientific rigor is what the effect of it should be. Rule out that increasing chronic election fraud is not the cause of the anomalies we see between post election polling and actual results.
I use scientific experts in all my cases that go to trial. Whether they are economic experts, or medical experts, or engineering experts, or some other techinical expert, they all rely on statistics and probablities. They all must pass the junk science requirement.
All I ask is that people who want to use design effect theory as a basis to justify an anomaly have thoroughly proven it to be the cause of the anomaly and not just a convienient excuse for it. When I see numbers like 50-80%, and when I see rapid increases in the numbers, I get suspicious.
Show me the peer-reviewed academic publications done by people who have no financial or other interest in the matter and you will make a believer out of me.
Posted by: davidgmills | Dec 15, 2004 9:59:37 AM
M.P., thanks for the link the Bill Kaminsky's blog. Loved the chart he posted. And I absolutely love this site too, and I can't thank you enough for all the work you put into it and all the great insights.
But I have a few quibbles with your second point today, "2) Considering sampling error, did the end-of-day numbers predict a Kerry victory?". First, you were right on target with taking Freeman to task over ignoring systematic bias, mainly for the reason you gave quite a while back: the magnitude of the conspiracy required at so many different levels of the process and in so many disparate locations in order for fraud to be the explanation for the discrepency between the exit polls and the actual vote is unfathomable and just not believable. So given that, and given the high emotions around this election, I don't think it's prudent to do any "what if" analysis that ignores the systematic error. My second issue is with what you actually did in response to your question. I think that what your analysis showed was not really an emphatic "No, not even close", but rather "not absolutely, but the exit polls do predict the odds of a Kerry win to be X%" -- where X is a number that is probably greater than 90%. The confidence intervals you use to establish the margin of error are irrelevant and in this case misleading. What you really need is the probability that Kerry could have lost and still hit the exit poll numbers that he did if you assume no bias in the exit polls. (I think that what you need to do is to integrate the binomial distribution (let's ignore 3rd parties) over all possible values of true means p that have Kerry losing the election, with the actual exit poll numbers plugged in -- perhaps some of the statisticians can help us out here?) I would bet that you'd come up with something crazy and close to what Freeman did, which is why I wouldn't touch this topic in the first place based on just the statistics of the exit polls. The more important questions are those you alluded to early on -- why was there so much bias in the exit polls?
Posted by: Scott W. | Dec 15, 2004 10:31:34 AM
There is a good reason why statisticians are often not considered good or reliable social scientists. Statistics is a means and not an end, in and of itself it does not resolve any problems, it just points in directions where you need to look. This is why statisticians are used for the most part as "consultants" in conducting social science research. None of the great social scientists that I know of were great or even very good statisticians.
First, whether you consult mathemeticians or statisticians. If you are looking for a statistical package to examine a problem you ask a statistician. If you are looking for the legitimacy of a statistical problem you approach a mathematician. Statistics at their base are still mathematics.
What social scientists do is locate the issue, see if there is a problem, and then try to determine a rational answer for the problem. By rational you are exploring how and why this problem occurred. What is critical is that you maintain this search for a rational answer. Right now the response that it was design effect IS NOT RATIONAL IN A SOCIAL SCIENCE SENSE. This is because the design effect has not been 1) Described in any detail, 2) Explained as to why it occurred, 3) Explained as to why it did not occur in previous situations. The idea of design effect is, as far as I can tell, being used in a RELIGIOUS or a MAGICAL THINKING context that Levi-Strauss described as PRE-RATIONAL when comparing societies and Piaget described as PRE-OPERATIONAL when talking comparing ages. It is roughly akin to being asked why the sun is always behind your back and saying it is because it is always following you. It is SYNCRETIC thinking, believing that A must have been the reason because B was the product.
There is a way to solve this and that is by having scientists look at the raw exit polling data. The idea that this is not being released, not even "unofficially" may be the most frightening aspect of everything that is going on right now.
Posted by: Wilbur | Dec 15, 2004 11:47:36 AM
Rick - thanks for the explanation. My problem is:
1. The Mystery pollster seems to believe that the rules concerning "significant digits" should be applied as a correction to Freeman's data, even though p and q are perfectly anti-correlated, which invalidates the error propagation formula ("significant digit" rules of summing,etc. assume no covariance between the numbers being added, subtracted, multiplied, or divided).
What's that in simple terms? If Bush has in reality 50.49% and Kerry has 49.51% then rounding takes a nearly 1% lead and makes it 0%. This is *always* biased against the real winner. So what's this about no covariance and no bias in estimators and randomly distributed whazzits? Real error propagation formulas start like this:
sqrt( (p + dp + dk)*(1 - p - dp - dk) )
and calculate the first and second order effects of uncertainties in p (dp) and roundoff error (dk). Formulas connecting sample Nb (Number voting for Bush) and Pb (popululation total voting for Bush) had also better be correlated.
Now that's a bias that generates the effect attributed to fraud and isn't a model error -- shouldn't someone have thought of that by now? It's not only a matter of imprecision in calculation, it's a matter of built in bias.
2. Rick, you seem to think a two tailed Z test and a t test are the same thing. (Can't possibly be true--the t distribution has an extra parameter, degrees of freedom). At a minimum there are some unwarranted assumptions in that, that need to be spelled out.
3. In any event, no one who does not understand why an arcsin transformation might be relevant to p and q this discussion (including me--I was wrong about the logarithms :) ) should be passing themselves off on the web as a statistician. Even we simple physicists know that.
So my question is, as someone pointed out above, if workaday scientists can see through the best arguments being circulated on the web, as far too naive, why can't we simply have competent ones? So far we've had a gaggle of social science grad students who have too much software and too little statistics background for their own good. (Berkeley? Bwahahaha. Freeman--looks like propaganda to me).
What I would expect from a *real* argument is either a competent discussion starting with linear statistics and explaining the assumptions that go into it (with or without justification, and certainly with lots of plots showing residuals, quantile-quantile plots, estimated deviations from normality and crap). Freeman's paper falls *way* short of that, and the "refutation" here, while interesting, could be better.
Or they should be talking about approximations to binomial distributions, hypergeometric functions (like defect samples in quality assurance), type I and type II errors, and something called "operational characteric" curves. I understand that kind of stuff doesn't show up on your typical blog, but a link to a PDF file tucked away on an academic server somewhere?
Why can't the polling agencies simply publish a graph of their OC curves for their sampling, same as any lowly QA inspector of war materiel would be required to do? Then some of us will know and the rest of us can go re-learn stats.
For a binomial distribution, there is a vast body of relevant literature having to do with random walks, expected number of zero-crossings, sign-flip tests, and censored samples, etc., yet I am seeing none of that dragged in. There should be some really cool bounds or something that are a lot more relevant.
My problem is, I'm not seeing the public discussion by statisticians I think I should be seeing. On either side.
Posted by: John Goodwin | Dec 15, 2004 11:52:00 AM
"All I ask is that people who want to use design effect theory as a basis to justify an anomaly have thoroughly proven it to be the cause of the anomaly and not just a convienient excuse for it. When I see numbers like 50-80%, and when I see rapid increases in the numbers, I get suspicious."
davidgmills, do you know that the design effects given to me by Mitofsky and the margin of error table were estimated PRIOR to the election?
re: your question about peer-reviewed discussion of the "design effect" - there is little on the design effect associated with EXIT POLLS. But there is an entire body of literature about the added error resulting from clustering and stratification of samples. The statistical theory behind the design effect "phenomenon" is solid and well documented, but the design effect as directly to exit polls of presidential elections is not. That is, we can quibble over the actual degree of the design effect (say that Mitofsky, Merkle, and PAnagakis are lying or grossly miscalculated), but to question the existence of the de is frankly absurd. I wish I had the time to write a literature review on this subject... Perhaps if you wait a couple months...
GREAT response. re: the Z-test v. t-test. You are technically correct, but my stats prof (maybe he is wrong) says the field is moving to a uniform t-test. The next edition of his book on Survey Research Methods will refer to Z-tests as the same thing as a t-test because he says although in theory they are different, in practice they are the "functionally" the same thing. This goes to a long-time debate between statistical theory purists and practioners.
I have a final today in Survey Research Methods. I will try and respond to some of your points later and I would like clarification on other points. BTW - I am not a statitiscian (duh), but as you say, a grad student with too much software and enough stats background (I do have extensive social science stats coursework behind me) to get me in trouble.
I want to refine my "approximation", but maybe I don't know enough to do it competently and cogently? Is that what you are saying? I won't be offended, I just take what I've learned and apply. If its wrong, then I can learn more. That's what life is all about right?
The problem I have I suppose is that if there was a way to arrive at very refined statistical conclusions given the available data, I would think someone would have done it by now... Maybe they are doing it, but they aren't saying anything until the article is published. I do have one peer-reviewed journal article under my belt, so I know the process. Anyone can post something on a blog.
AGAIN - this is all academic. Mitofsky and Lenski say that there is a proportion of the discrepancy that they cannot explain by sampling error. They think it has to do with faulty methods or differential non-response. Others say that Bush stole the election. Who knows? I think that I'm just agreeing with Wilbur above... Stats have limitations... They don't "prove" anything. Even rejection of a null hypothesis can lead to spurious conclusions.
Posted by: Rick Brady | Dec 15, 2004 1:26:03 PM
"However it assumes that Z-scores and p-values are linear functions of the proportions, which is not true, so I don't think it's necessarily the best approach."
The Z-scores were determined by the following equation:
p=proportion from exit poll; P=proportion from election result.
The s.e. of the exit poll was determined by taking the standard error of a srs (s.e. = SQRT(PQ/n) based on the sample size of the exit poll and multiplied by a factor of 1.6. 1.6 is what Mitofsky estimated that the square root of the design effect would be for states where the average number of samples drawn per precinct sampled was 40. It was exactly 40 in Ohio, therefore the 1.6 factor is appropriate.
Now how does this assume that the "Z-scores and p-values are linear functions of the proportions?" I do realize that the "actual" design effect for Ohio is likely different than the "estimated" design effect, but I am using what Mitofsky, Merkle, and Edelman estimated for the design effect PRIOR to the election. These design effects were then used by Mitofsky to build the Margin of Error table recently published by the NEP and circulated with the exit poll data to the networks on election day. The refined estimates actually reduce the confidence intervals reported in the MoE table (well, mostly... there are some states where the rounding goes the other way, but these are the minority).
We don't have the data required to do a precinct by precinct analysis for a more refined design effect factor. At least, that is the way I see it. What do you think?
Posted by: Rick Brady | Dec 15, 2004 1:35:51 PM
Why would you beleive that systemic fraud is not likely? That is not what the computer hackers and experts are saying.
When 4 companies count 80% of America's vote on computer, in secret, and not by hand, and not even locally, why would you ever believe that systemic fraud is not a reasonable possibility? Almost every vote, even those on mechanical machines, are ultimately downloaded and sent to a computer to be counted.
These four companies are not even publicly held companies, nobody even knows who their major shareholders are, or what percent of their employees are American citizens, or what the criminal records of their employees are.
Posted by: davidgmills | Dec 15, 2004 1:44:34 PM
I have always assumed that the design effect is valid. It even makes sense to me that if clutering exists you have to account for it. But, I mentioned that one mathematician who has posted widely on the internet does not believe it exists. For the sake of argument, I think he is wrong.
Assuming it does exist there are two things:(1) if the sample is large enough does the clustering effect cancel itself out at some point, and (2) if it doesn't cancel itself out, what id the proper value for it.
As you said, scientists can "quibble" about the actual degree of it, and frankly that is my "quibble." How did the experts arrive at the actual degree (50-80%). Have they published their work in coming up with this degree of effect? Was it published in a respected journal? Was it peer reviewed? etc., etc. Were the authors biased due to financial interests? Were there conflicts of interest? I could go on and on.
Posted by: davidgmills | Dec 15, 2004 2:03:32 PM
While I have found the discussions in this forum fascinating (I am not being facetious), most of it is a complete waste of time, for the most part. I am willing to grant that the exit polls seem to have a bias that favored John Kerry, and this points to a systemic flaw in the methodology, either in the collection of the data or in the models used for refinement. However, the issue that is foremost at this point is to have all of the raw data released, the identities of the precincts polled. At that point we no longer will have to completely guess at the flaws.
Posted by: Yancey Ward | Dec 15, 2004 2:10:36 PM
I can add this to the discussion now:
Dan Merkle of ABC and the co-author of the chapter that analyzed the 1996 exit polls and determined that the design effect square root of that exit poll was 1.3 has gone on record with the following statement:
"What was in the Merkle and Edelman chapter is only a general estimate based on work at VNS in the early 1990s. The design effect will vary state by state based on the number of interviews per precinct and how clustered the variable is. More work was done on this in 2004 by Edison/Mitofsky. Edelman and I did participate in this. I would suggest using the design effects calculated by Edison/Mitofsky for their 2004 polls."
The Ohio calculations above are based on the Edison/Mitofsky estimates of the design effect.
davidgmills, no they have not published their methods for the 2004 exit polls. I assume this will all become public once they complete a precinct by precinct analysis.
However, there is a table in the Merkle-Edelman chapter that does suggest that as the number of samples in the 1996 poll increased, the difference between the standard error of a simple random sample and a cluster sample of the same sample size decreased. That is, as sample size increases, the design effect should decrease. However, READ the Merkle quote above. You cannot take Merkle's findings, or any "peer reviewed" findings of a previous election and apply them to the 2004. We have to wait...
In the meantime, I think it's worthwile to point out that Dr. Freeman's analysis is HIGHLY flawed. That, I think, was the point of MPs post.
Posted by: Rick Brady | Dec 15, 2004 2:22:28 PM
Posted by: John Goodwin | Dec 15, 2004 4:55:52 PM
And as a final request, how about a comment on the Chebyshev inequality, which for Bernoulli trials is
P(|k/n -p |>eps) < pq/n/eps**2
and in general < sigma**2/eps**2
So for a given confidence level,
0.95 = 1 - p*q/1.8/N/eps**2 (if you believe in
1.8 for clustering). Solve for epsilon.
A typical application: suppose you play a coin toss game with a loaded coin and wins/losses are a and b respectively,
what are bounds on your net gain with
some given confidence.
You don't need design parameters after
the election, since you *know* sigma.
Discussed in section 2.7-8 of these notes, for the mathematically inclined....
Posted by: John Goodwin | Dec 15, 2004 7:38:09 PM
John Goodwin. How about this. I give you a spreadsheet with the data pulled by Freeman from the CNN website around midnight on election eve and the election results for each state (sans Virginia - it's missing) and you run the numbers? If it's sooooo easy, then it shouldn't take any time at all. Then you can send me the spreadsheet back with your calcs. I'll try to make sense of them (and run them by my Stats prof) and see if I agree with your methods.
Look - you seem to be saying you have a better way of answering the questions both Freeman, myself and others are trying to answer, so put your money where your mouth is and SHOW ME THE MONEY!!!!
I got the data. You got the brains. Together we can make scrambled eggs and toast. Okay?
My email is rick at alohalee dot com
You e-mailed yet? What gives? Come on...
It seems as if you don't really want a statistical answer to this question or you would have asked for the data by now and run your own tests. That makes me think you are either blowing hot air, or are just trying to make us all look stupid.
I won't say I'm stupid, but I will say that I don't know everything about stats. Okay, maybe I am stupid compared to you. I just take what I learned in my graduate and undergaduate level courses and apply... Why teach the stuff if it's not meant to be applied? Why take a course in survey research methods from a prof who: 1) owns his own polling firm; and 2) designed several cluster sample based polls for transit bus riders (randomly sampled buses and intercepted passengers) - if that prof can't tell me I'm on the right track with my analysis?
Like I said, I'm just learning. Maybe I'm being fed a bunch of BS. TRUST ME - it wouldn't be the first time a prof fed me a load of crap.
I'm teachable - so teach me. Please?
Posted by: Rick Brady | Dec 15, 2004 8:47:22 PM
Cool Rick. Just saw your post and mailed you.
My comments with requests for clarification were to MP (since this is his blog), and weren't directed to you. Just figured out that you = stones-cry-out so let's move to your blog.
Posted by: John Goodwin | Dec 15, 2004 9:36:47 PM
Wilbur: OK, I'll take the bait. Being a statistician, I'm clearly biased. But I think most statisticians realise that mathematicians usually do not have a good practical grasp of key statistical principles such as the importance of understanding variation, data collection procedures and bias/variance trade-offs. Mathematics provides a theoretical foundation for statistics, in the same way that formal logic provides a foundation for mathematics, but I wouldn't expect a logician to give good advice on a hard math problem.
Part of the problem here is that the word "statistician" can mean a wide range of things: a statistical clerk, a baseball fan, a rote analyst who knows one statistical package, a professional pollster, or a Nobel prize winning professor.
Posted by: James Reilly | Dec 16, 2004 3:52:20 AM
Rick, the Z-score (p-P)/s.e.(p) is non-linear because s.e.(p) is a function of p. For sampling with replacement, Z=(p-P)/sqrt(p(1-p)/n) which is a non-linear function of p. But as I said, it's probably not highly non-linear over the ranges of interest, so I expect your approach above is fine.
Posted by: James Reilly | Dec 16, 2004 3:56:48 AM
I have some friends who do advanced research in mathematics and I'm always amazed that there are no numbers involved. Mathematics in its purest form does revert back to logic. I'm not sure there are any such things as pure logicians - logic is now part of analytical philosophy but that moves away from the more mathematical logic you are talking about.
The trouble with statisticians is that they create rules because unlike mathemeticians who work everything out to the nth degree they have to move fast, but then they forget that rules are created for a specific context and put the rule before the context.
I am neither mathematician or statistician - just a plain old social scientist. But some of my best friends are mathematicians so I too am biased, but let me tell you a little story anyway - very basic but very telling. I was working out an issue and I used a Chi-Square, mostly because I didn't know what I was doing I used it for repeated measures. The findings were significant at .01 or something like that. The statisticians were up in arms saying that you can't use a Chi-Square for repeated measures because it was against the rules. I took my problem to a very good mathematician who looked into the math behind what I was using. She told me that the reason you can't use a Chi-Square in repeated measures is because it works against your interest in finding a significant difference, but if you do find a significant difference it is actually much more powerful. Logically there should be no argument against it. A bit nervous I went to a second, more eminent mathematician and he backed this up. I took it back to the statisticians, but not matter how much I made the mathematical/logical argument they said it was against the rules. It bothers me to this day. So math is a corrupted version of logic, and statistics is a corrupted version of math, but they exist for their own uses. Still when you find problems in statistics you go back to the math, and when you find problems in math you go back to the logic. What this has to do with exit polling I don't know.
Posted by: Wilbur | Dec 16, 2004 10:00:05 AM
As a statistician I'd like to weigh in on a few things mentioned here so far.
1) When you ask for "proof" of statistical theory, all you're going to get is more statistical theory. That's why we have theorems and not facts, but these theorems are backed up by decades upon decades of statistical research and studies and are widely accepted as "true" by all statisticians.
2) The design effect is higher when the cluster size is higher because the more data you put into a cluster, the less likely it is that the cluster is representative of the population as a whole. The idea is of clustering is to mimic a random sample, not replace it, and thus there is more error introduced because of that.
3) No one has yet mentioned the biggest error (in my opinion) of Dr. Freeman's paper and that is his 1 in 662,000 calculation. To get this number, he simply multiplied the p-values from OH, FL, and PA together. The problem here is that a p-value is not a measure of the probability of an event occurring but a measure of the evidence for or against the null hypothesis. Because of this, you cannot multiply p-values together and get a meaningful number. To test this out, just multiply the p-values from all 51 states and you'll see my point.
4) I'm also wary of the argument about rounding that's going on. I've run my analysis using both rounded numbers and unrounded numbers and the results don't appear to be all that different.
I plan on writing a paper discussing my analysis in the coming days and I'll be sure to post it here if I get the chance.
Posted by: c-macdonald | Dec 16, 2004 3:31:42 PM
The comments to this entry are closed.