Cover your wallet when you cough...

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Scientists exploring the way disease spreads among humans have found a treasure trove of data from the site Where's George. From the introductory note at Nature:

Analysis of the trajectories of over half a million dollar bills shows that human dispersal is described by a 'two-parameter continuous-time random walk' model: our travel habits conform to a type of random proliferation known as 'superdiffusion'. And with that much established, it should soon be possible to develop a new class of models to account for the spread of human disease.

The only nit I can think to pick here is to question whether there is a difference in travel patterns between the average person and those 1)more likely to use a good deal of cash and 2) are likely to enter their dollar bill serial numbers into a website. This may, of course, be addressed in the article, to which I do not have access other than the first graf:

The dynamic spatial redistribution of individuals is a key driving force of various spatiotemporal phenomena on geographical scales. It can synchronize populations of interacting species, stabilize them, and diversify gene pools1, 2, 3. Human travel, for example, is responsible for the geographical spread of human infectious disease4, 5, 6, 7, 8, 9. In the light of increasing international trade, intensified human mobility and the imminent threat of an influenza A epidemic10, the knowledge of dynamical and statistical properties of human travel is of fundamental importance. Despite its crucial role, a quantitative assessment of these properties on geographical scales remains elusive, and the assumption that humans disperse diffusively still prevails in models. Here we report on a solid and quantitative assessment of human travelling statistics by analysing the circulation of bank notes in the United States. Using a comprehensive data set of over a million individual displacements, we find that dispersal is anomalous in two ways. First, the distribution of travelling distances decays as a power law, indicating that trajectories of bank notes are reminiscent of scale-free random walks known as Lévy flights. Second, the probability of remaining in a small, spatially confined region for a time T is dominated by algebraically long tails that attenuate the superdiffusive spread. We show that human travelling behaviour can be described mathematically on many spatiotemporal scales by a two-parameter continuous-time random walk model to a surprising accuracy, and conclude that human travel on geographical scales is an ambivalent and effectively superdiffusive process.

Hey -- did they say "decays as a power law"? Chris Anderson, please call your office. Someone would like to talk to you about the long tail of disease. (Easy to identify: the specialization of docs and drugs to ease the million small ailments that plague everyday life. Harder to assess: Since we don't truly face the costs of our own care in the developed world, it's a redistributive requirement that these ailments get treated, since they are more prominent as people age. And as social investment goes, old folks are nice, but see little to no return as compared to getting kids healthy and educated. If the money is spent -- distorted by the odd employer-based insurance program and the public funding system we have -- primarily on old age problems, then its in those specialties where the doctors will concentrate. Advancement in geriatric care at the expense of early preventative care. Are we still happy with the long tail when it's communal?)


NB: Nod to the FRIAM group's discussion list for the pointer.

3 Comments

In recent years, there has been a lot of hullabaloo in natural sciences journals-- mostly people in evolutionary biology who apply their Bayesian statistical methods to historical linguistics. They believe that their models for evolution more closely mimic the paths that languages have traveled than linguistics methods (leaving aside for a moment that such methods have been employed by linguists, and leaving aside the fact that their models may mimic but they do not explain whatsoever)--

I've always felt that money and language are two of the great human constants in this universe. That is, our needs to exchange and speak. We are able to quantify and study both these things, find a corpus and study it for each... it stands to reason, to me, that data from Where's George and similar experiments could provide for really interesting findings in historical and sociolinguistics too. Such linguistic endeavors are as fast and flexible as the immunological ones, and possibly even more complex.

I've had a wheresgeorge dollar sitting on my desk for over a year awaiting sufficient boredom or sufficient need of a single as to inspire me to log its location.

Frankly, I'm surprised that WG has worked. Individually, the information on a bill is pretty boring. Is it a surprise that a dollar bill went from Montana to NYC? I can't muster enough interest in logging a bill, let alone tracking where it went. But in the aggregate, obviously it's fascinating. I didn't even know what a "superdiffusive" process was. (Not sure I really do now, either.)

On a similar note, has anyone, ever, marked a bill with pen of some sort and actually SEEN the bill again? (I'm assuming that's the intent of people who blacken corners, or who write codes or names on the edges. Perhaps there are just a lot of bored people who have a pen and a bill.)

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