First of all, I don't know that there's a reason to suspect a bias from a uniform 1/72 distribution. If you see 100 plates per day, the expected number of times the most common two-letter code appears is something like 4.8, so seeing a particular plate 5 times seems unexceptional. Further, I expect a slight bias toward newer plates since there is some turnover, especially from heavily-used vehicles. This is particularly relevant to CW which should, by that measure, be one of the most common. (Also, if we're expecting some fraction of vehicles to be driven often enough that they need to be retired in < 4 years, those must be driven a disproportionately large share of the time, further increasing the bias toward later letter combinations.) Charles Greathouse Analyst/Programmer Case Western Reserve University On Mon, Sep 23, 2013 at 7:00 AM, Simon Plouffe <simon.plouffe@gmail.com>wrote:
Hello,
I live in france now and they have a plate numbering system that puzzles me a little bit.
Here are some explanations about a new system : http://en.wikipedia.org/wiki/Vehicle_registration_plates_of_France
roughly, the plates are sequenced with letters and digits: in the following way.
AA 123 XX
where letters are a-z except I O and U for various reasons. There are some specific restrictions on some specific 2 letters combination , but we can consider this factor being negligible.
So, they began this numbering scheme in march 2009. Beginnin with AA, for now the letters are at CY,
So to make it simple there are 2 types of plates you can encounter by random.
1) the current numbering system 2) the old system + the foreigns plate that we do not count.
So , for my car the first letters are CG this means there is about 1 chance out of 72 (rejecting the plates of type 2). that I encounter a car with these same letters. And consequently, there is 1 chance in 72 that I encounter CW or BT etc. Since there are 24 letters for each and they covered 3 letters so far then there are 72 combinations.
BUT the problem is the following, this 1 chance out of 72 does not follow statistical distribution at all. , like this morning I encountered 5 vehicles with the prefix CW and I really can't explain why, I calculated that in one day I could see and analyse maybe 100 cars with plate numbers but each day the basic statistics does not add up at all .
I am scratching my head and I cannot see why there is a strong statistical bias in the distribution of those 2 letter prefixes.
Can anybody ever encountered such bias.
By the way, this method of analysis was used during WWII , they where doing reverse engineering on the serial numbers found on tanks and planes to know how many the enemy might have, , they eventualy changed to a more cryptic numbering system.
Best regards,
Simon Plouffe _______________________________________________ math-fun mailing list math-fun@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/math-fun