## Unusual timing shows how random mass murder can be (or even less)

Posted in Books, R, Statistics, Travel with tags , , , , , , , , on November 29, 2013 by xi'an

This post follows the original one on the headline of the USA Today I read during my flight to Toronto last month. I remind you that the unusual pattern was about observing four U.S. mass murders happening within four days, “for the first time in at least seven years”. Which means that the difference between the four dates is at most 3, not 4!

I asked my friend Anirban Das Gupta from Purdue University are the exact value of this probability and the first thing he pointed out was that I used a different meaning of “within 4″. He then went into an elaborate calculation to find an upper bound on this probability, upper bound that was way above my Monte Carlo approximation and my rough calculation of last post. I rechecked my R code and found it was not achieving the right approximation since one date was within 3 days of three other days, at least… I thus rewrote the following R code

T=10^6
four=rep(0,T)
for (t in 1:T){
day=sort(sample(1:365,30,rep=TRUE)) #30 random days
day=c(day,day[day>363]-365) #account for toric difference
tem=outer(day,day,"-")
four[t]=(max(apply(((tem>-1)&(tem<4)),1,sum)>3))
}
mean(four)

[checked it was ok for two dates within 1 day, resulting in the birthday problem probability] and found 0.070214, which is much larger than the earlier value and shows it takes an average 14 years for the “unlikely” event to happen! And the chances that it happens within seven years is 40%.

Another coincidence relates to this evaluation, namely the fact that two elderly couples in France committed couple suicide within three days, last week. I however could not find the figures for the number of couple suicides per year. Maybe because it is extremely rare. Or undetected…

## Unusual timing shows how random mass murder can be (or not)

Posted in Books, R, Statistics, Travel with tags , , , , , , , , on November 4, 2013 by xi'an

This was one headline in the USA Today I picked from the hotel lobby on my way to Pittsburgh airport and then Toronto this morning. The unusual pattern was about observing four U.S. mass murders happening within four days, “for the first time in at least seven years”. The article did not explain why this was unusual. And reported one mass murder expert’s opinion instead of a statistician’s…

Now, there are about 30 mass murders in the U.S. each year (!), so the probability of finding at least four of those 30 events within 4 days of one another should be related to von Mises‘ birthday problem. For instance, Abramson and Moser derived in 1970 that the probability that at least two people (among n) have birthday within k days of one another (for an m days year) is

$p(n,k,m) = 1 - \dfrac{(m-nk-1)!}{m^{n-1}(m-nk-n)!}$

but I did not find an extension to the case of the four (to borrow from Conan Doyle!)… A quick approximation would be to turn the problem into a birthday problem with 364/4=91 days and count the probability that four share the same birthday

${30 \choose 4} \frac{90^{26}}{91^{29}}=0.0273$

which is surprisingly large. So I checked with a R code in the plane:

T=10^5
four=rep(0,T)
for (t in 1:T){
day=sample(1:365,30,rep=TRUE)
four[t]=(max(apply((abs(outer(day,day,"-"))<4),1,sum))>4)}
mean(four)

and found 0.0278, which means the above approximation is far from terrible! I think it may actually be “exact” in the sense that observing exactly four murders within four days of one another is given by this probability. The cases of five, six, &tc. murders are omitted but they are also highly negligible. And from this number, we can see that there is a 18% probability that the case of the four occurs within seven years. Not so unlikely, then.

## 2013 WSC, Hong Kong

Posted in Books, pictures, Running, Statistics, Travel, University life with tags , , , , , , , , , on August 28, 2013 by xi'an

After an early but taxing morning run overlooking the city, and a recovery breakfast (!), I went from my flat to the nearby Hong Kong Convention Centre where the ISI (2013 WSC) meeting is taking place. I had a few chats with friends and publishers (!), then read a chapter of Rissanen’s book over an iced coffee before attending the Bernoulli session. This was a fairly unusual session with a mix of history of probability, philosophy of probability and statistics, and computational issues (my talk). Edith Sylla gave some arguments as to why Ars Conjectandi (that she translated) was the first probability book ever. Krzys Burdzy defended his perspective on why von Mises and de Finetti were wrong (in their foundational views of statistics). And I gave my talk on a mixture of Bernoulli factory, Russian roulette and ABC  (After my talk, Victor Perez Abreu told me that Jakob Bernoulli had presumably used simulation to evaluate the variance of the empirical mean in the Bernoulli case.) What I found most interesting in the historical talk was that Bernoulli had proven his result in the late 1680’s but he waited to complete his book on moral and commercial issues, waited too long since he died before. This reminded me of Hume using probabilistic arguments a few years later to disprove the existence of miracles. And of Price waiting for Bayes’ theorem to counter Hume. The talk by Krzys was a quick summary of the views exposed in his book, which unsurprisingly did not convince me that von Mises and de Finetti (a) had failed and (b) needed to use a new set of (six) axioms to define probability. I often reflected on the fact that when von Mises and de Finetti state(d) that probability does not exist, they applied the argument to a single event and this does not lead to a paradox in my opinion. Anyway, this talk of Krzys’ induced most of the comments from the floor, my own talk being in fine too technical to fit in this historical session. (And then there was still some time to get to a tea shop in Sheng Wan to buy some Pu Ehr, if not the HK\$3000 variety…!)

## Decision systems and nonstochastic randomness

Posted in Books, Statistics, University life with tags , , , , , on October 26, 2011 by xi'an

Thus the informativity of stochastic experiment turned out to depend on the Bayesian system and to coincide to within the scale factor with the previous “value of information”.” V. Ivanenko, Decision systems and nonstochastic randomness, p.208

This book, Decision systems and nonstochastic randomness, written by the Ukrainian researcher Victor Ivanenko, is related to decision theory and information theory, albeit with a statistical component as well. It however works at a fairly formal level and the reading is certainly not light. The randomness it address is the type formalised by Andreï Kolmogorov (also covered in the book Randomness through Computation I [rather negatively] reviewed a few months ago, inducing angry comments and scathing criticisms in the process). The terminology is slightly different from the usual one, but the basics are those of decision theory as in De Groot (1970). However, the tone quickly gets much more mathematical and the book lost me early in Chapter 3 (Indifferent uncertainty) on a casual reading. The following chapter on non-stochastic randomness reminded me of von Mises for its use of infinite sequences, and of the above book for its purpose, but otherwise offered an uninterrupted array of definitions and theorems that sounded utterly remote from statistical problems. After failing to make sense of the chapter on the informativity of experiment in Bayesian decision problems, I simply gave up… I thus cannot judge from this cursory reading whether or not the book is “useful in describing real situations of decision-making” (p.208). It just sounds very remote from my centres of interest. (Anyone interested by writing a review?)

## Error and Inference [#4]

Posted in Books, Statistics with tags , , , , , , , , , , , , , , on September 21, 2011 by xi'an

(This is the fourth post on Error and Inference, again and again yet being a raw and naïve reaction following a linear and slow reading of the book, rather than a deeper and more informed criticism.)

‘The defining feature of an inductive inference is that the premises (evidence statements) can be true while the conclusion inferred may be false without a logical contradiction: the conclusion is “evidence transcending”.”—D. Mayo and D. Cox, p.249, Error and Inference, 2010

The seventh chapter of Error and Inference, entitled “New perspectives on (some old) problems of frequentist statistics“, is divided in four parts, written by David Cox, Deborah Mayo and Aris Spanos, in different orders and groups of authors. This is certainly the most statistical of all chapters, not a surprise when considering that David Cox is involved, and I thus have difficulties to explain why it took me so long to read through it…. Overall, this chapter is quite important by its contribution to the debate on the nature of statistical testing.

‘The advantage in the modern statistical framework is that the probabilities arise from defining a probability model to represent the phenomenon of interest. Had Popper made use of the statistical testing ideas being developed at around the same time, he might have been able to substantiate his account of falsification.”—D. Mayo and D. Cox, p.251, Error and Inference, 2010

The first part of the chapter is Mayo’s and Cox’ “Frequentist statistics as a theory of inductive inference“. It was first published in the 2006 Erich Lehmann symposium. And available on line as an arXiv paper. There is absolutely no attempt there to link of clash with the Bayesian approach, this paper is only looking at frequentist statistical theory as the basis for inductive inference. The debate therein about deducing that H is correct from a dataset successfully facing a statistical test is classical (in both senses) but I [unsurprisingly] remain unconvinced by the arguments. The null hypothesis remains the calibrating distribution throughout the chapter, with very little (or at least not enough) consideration of what happens when the null hypothesis does not hold.  Section 3.6 about confidence intervals being another facet of testing hypotheses is representative of this perspective. The p-value is defended as the central tool for conducting hypothesis assessment. (In this version of the paper, some p’s are written in roman characters and others in italics, which is a wee confusing until one realises that this is a mere typo!)  The fundamental imbalance problem, namely that, in contiguous hypotheses, a test cannot be expected both to most often reject the null when it is [very moderately] false and to most often accept the null when it is right is not discussed there. The argument about substantive nulls in Section 3.5 considers a stylised case of well-separated scientific theories, however the real world of models is more similar to a greyish  (and more Popperian?) continuum of possibles. In connection with this, I would have thought more likely that the book would address on philosophical grounds Box’s aphorism that “all models are wrong”. Indeed, one (philosophical?) difficulty with the p-values and the frequentist evidence principle (FEV) is that they rely on the strong belief that one given model can be exact or true (while criticising the subjectivity of the prior modelling in the Bayesian approach). Even in the typology of types of null hypotheses drawn by the authors in Section 3, the “possibility of model misspecification” is addressed in terms of the low power of an omnibus test, while agreeing that “an incomplete probability specification” is unavoidable (an argument found at several place in the book that the alternative cannot be completely specified).

‘Sometimes we can find evidence for H0, understood as an assertion that a particular discrepancy, flaw, or error is absent, and we can do this by means of tests that, with high probability, would have reported a discrepancy had one been present.”—D. Mayo and D. Cox, p.255, Error and Inference, 2010

The above quote relates to the Failure and Confirmation section where the authors try to push the argument in favour of frequentist tests one step further, namely that that “moderate p-values” may sometimes be used as confirmation of the null. (I may have misunderstood, the end of the section defending a purely frequentist, as in repeated experiments, interpretation. This reproduces an earlier argument about the nature of probability in Section 1.2, as characterising the “stability of relative frequencies of results of repeated trials”) In fact, this chapter and other recent readings made me think afresh about the nature of probability, a debate that put me off so much in Keynes (1921) and even in Jeffreys (1939). From a mathematical perspective, there is only one “kind” of probability, the one defined via a reference measure and a probability, whether it applies to observations or to parameters. From a philosophical perspective, there is a natural issue about the “truth” or “realism” of the probability quantities and of the probabilistic statements. The book and in particular the chapter consider that a truthful probability statement is the one agreeing with “a hypothetical long-run of repeated sampling, an error probability”, while the statistical inference school of Keynes (1921), Jeffreys (1939), and Carnap (1962) “involves quantifying a degree of support or confirmation in claims or hypotheses”, which makes this (Bayesian) sound as less realistic… Obviously, I have no ambition to solve this long-going debate, however I see no reason in the first approach to be more realistic by being grounded on stable relative frequencies à la von Mises. If nothing else, the notion that a test should be evaluated on its long run performances is very idealistic as the concept relies on an ever-repeating, an infinite sequence of identical trials. Relying on probability measures as self-coherent mathematical measures of uncertainty carries (for me) as much (or as less) reality as the above infinite experiment. Now, the paper is not completely entrenched in this interpretation, when it concludes that “what makes the kind of hypothetical reasoning relevant to the case at hand is not the long-run low error rates associated with using the tool (or test) in this manner; it is rather what those error rates reveal about the data generating source or phenomenon” (p.273).

‘If the data are so extensive that accordance with the null hypothesis implies the absence of an effect of practical importance, and a reasonably high p-value is achieved, then it may be taken as evidence of the absence of an effect of practical importance.”—D. Mayo and D. Cox, p.263, Error and Inference, 2010

The paper mentions several times conclusions to be drawn from a p-value near one, as in the above quote. This is an interpretation that does not sit well with my understanding of p-values being distributed as uniforms under the null: very high  p-values should be as suspicious as very low p-values. (This criticism is not new, of course.) Unless one does not strictly adhere to the null model, which brings back the above issue of the approximativeness of any model… I also found fascinating to read the criticism that “power appertains to a prespecified rejection region, not to the specific data under analysis” as I thought this equally applied to the p-values, turning “the specific data under analysis” into a departure event of a prespecified kind.

(Given the unreasonable length of the above, I fear I will continue my snailpaced reading in yet another post!)

## von Mises lecture [fertig]

Posted in Statistics, University life with tags , , , , on July 1, 2011 by xi'an

Here are the slides I prepared for my von Mises lecture today, managing to include some quotes from von Mises in the initial slides…

## Hotel with a blog

Posted in pictures, Travel, University life with tags , , on June 29, 2011 by xi'an

For my visit to Berlin tomorrow, I am staying in the Circus Hotel, which happens to have a true blog…! Amazing! Looking forward my visit to Berlin…