## Le Monde rank test

**I**n the puzzle found in ** Le Monde** of this weekend, the mathematical object behind the silly story is defined as a pseudo-Spearman rank correlation test statistic,

where the difference between the ranks of the paired random variables and is in absolute value instead of being squared as in the Spearman rank test statistic. I don’t know whether or not this measure of distance has been studied in the statistics literature (although I’d be surprised has it not been studied!). Here is an histogram of the distribution of the new statistics for under the null hypothesis that both samples are uncorrelated (i.e. that the sequence of ranks is a random permutation). Each point in the sample was obtained by

perm=sample(1:20) saple[t]=sum(abs(perm[1:10]-perm[11:20]))

When regressing the mean of this statistic against the covariates and , I obtain the uninspiring formula

which does not translate into a nice polynomial in !

**A**nother interesting probabilistic/combinatorial problem issued from an earlier ** Le Monde **puzzle: given an urn with white balls and black balls that is sampled without replacement, what is the probability that there exists a sequence of length with the same number of white and black balls for ? If , the answer is obviously one (1), but for some values of , it is less than one. When goes to infinity, this is somehow related to the probability that a Brownian bridge crosses the axis in-between and but I have no clue whether this helps or not! Robin Ryder solved the question for the values and by establishing that the probability is still one.

**Ps-** The same math tribune in ** Le Monde** coincidently advertises a book,

**, by Benoît Rittaud that adresses … climate change issues and the “statistical mistakes made by climatologists”. The interesting point (if any) is that Benoît Rittaud is a “mathematician not a statistician”, with a few papers in ergodic theory, but this advocated climatoskeptic nonetheless criticises the use of both statistical and simulation tools in climate modeling. (“Simulation has only been around for a few dozen years, a very short span in the history of sciences. The climate debate may be an opportunity to reassess the role of simulation in the scientific process.”)**

*Le Mythe Climatique*
April 5, 2010 at 10:13 am

[…] Monde rank test (cont’d) Following a comment from efrique pointing out that this statistic is called Spearman footrule, I want to clarify the […]

April 5, 2010 at 3:38 am

That’s an old statistic called “Spearman’s footrule”, and while it was pretty much ignored for many decades, recently it’s been used in particular ares of computing and more recently still in analysis of microarray data