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Statistical Anomaly

February 12, 2012

Interesting statistical anomaly.

Driving to work on the morning of Tuesday 25th October, I saw an interesting car number plate on the Bath Road, travelling towards Junction 12 on the M4. It was of the pattern: aaZ nnnn (where a denotes a letter and n a digit).

For those who don’t live in Britain and are unfamiliar with the registration plate system, here’s a quick overview.

Number plates of vehicles on the road at the moment are usually in one of two formats:
(1):  aann aaa
(2) : annn aaa

The digits in format (1) indicate the year of registration: either the last two digits of the year (from 02 to 11) or the same with 50 added (from 51 to 61). The latter format signifies the second half of that given year. Hence you can tell exactly how old a motor is by those two digits.

The first letter in format (2) also indicates the year of registration, for years before the second half of 2001: so Y indicates August 2000 to July 2001 and earlier letters indicate further back in time.

The rest of the letters indicate in some coded form the location where the vehicle was registered.

Well anyway, back to my story. Here was a vehicle with an unusual registration plate, and no indication where it came from. (Most foreign motors have a nation plate and the steering wheel on the wrong side. This one didn’t.) I gave it a good look as I went past it. It didn’t turn down the motorway, but instead took the exit to Theale. I noticed the driver: a young woman with big black hair.

Trundling down the M4, a few miles further on I did a double-take, as I saw what I thought was the same car again. It had what looked like the same number, anyway: aaZ nnnn. The first letter and the first digit were the same. Surely it can’t be the car that turned off towards Theale? So I stamped on the accelerator and hauled past this motor to look at the driver: a young woman, but this time with curly red hair. So it definitely was not the same motor. Well, well, I thought, two of these vehicles with odd number plates. Wonder how many more I’m going to see?

A few miles further on, there’s another one. Older and more scruffy, driven once again by a young woman, although this time she had a companion (another young woman). That’s three.

Then about ten miles further on, there’s a van, about Transit-sized, again with one of those weird number plates. That’s vehicle number 4. They must be all around after all. I didn’t get to look at the driver because at this time I’d reached Junction 14 and needed to turn off.

I got to work without seeing any more of these number plates – until I turned into our car park and there in front of the office next door is yet another of these number plates. That’s five of the things in one simple 26-mile journey of 40 minutes.

Five minutes research told me what I needed to know about the pattern aaZ nnnn.  Well, it turns out that such numbers indicate registration in Northern Ireland (technically part of the “United Kingdom of Great Britain and Northern Ireland”, to give my country its full name).  And some people have been known to use such numbers as vanity plates, as it is not easy to determine how old such a car is.

How much of a statistical anomaly is this?  How frequent are these registration plates seen on the road?  Well I’ve kept an eye out since.  And (apart from the one that parks for the office next door), I have never seen any more than the occasional one on any of my subsequent similar journeys.

Five (okay, technically four) in one journey, and none since? That’s what I call anomalous. Maybe there had been a convention attended by Northern Irish female students and they were all going home. Who knows?

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