## from tramway to Panzer (or back!)…

**A**lthough it is usually presented as *the tramway problem*, namely estimating the number of tram or bus lines in a city given observing one line number, including The Bayesian Choice by yours truly, the original version of the problem is about German tanks, Panzer V tanks to be precise, which total number *M* was to be estimated by the Allies from their observation of serial numbers of a number *k* of tanks. The Riddler is restating the problem when the only available information is made of the smallest, 22, and largest, 144, numbers, with no information about the number *k* itself. I am unsure what the Riddler means by “best” estimate, but a posterior distribution on *M* (and *k*) can be certainly be constructed for a prior like *1/k x 1/M²* on *(k,M)*. (Using M² to make sure the posterior mean does exist.) The joint distribution of the order statistics is

which makes the computation of the posterior distribution rather straightforward. Here is the posterior surface (with an unfortunate rendering of an artefactual horizontal line at 237!), showing a concentration near the lower bound M=144. The posterior mode is actually achieved for M=144 and k=7, while the posterior means are (rounded as) M=169 and k=9.

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