Just a minor pet peeve here. I was paying my cable bill today, and like the good consumer that I am, I wrote my account number on the check. SIXTEEN DIGITS! WTF? Does this make any sense at all? I mean I could understand if Cox Cable had a quadrillion customers, but unless there are creatures from distant galaxies who enjoy crappy service, poor channel selection, and high prices, I don’t think that this is the case.
My work involes entering a lot of purchase order numbers and things of that nature, so I see this all the time from many, MANY different companies. WHY? All it does is increase the chances of somebody, somewhere along the line, entering the wrong number.
If these companies were really smart, they’d use letters instead of numbers. That way you’d have a base 26 numbering system instead of base 10. That means fewer characters are required for the same number of customers. But that just makes too much sense, so I know it’ll never happen.
Neutron, you just answered your own question. Very long and unique numbers, widely separated in value, are LESS likely to be entered incorrectly, as whatever software they’re using will catch the error. It’s really for your protection, so I wouldn’t complain.
Now AMEX is offering ‘one use’ credit card numbers (for Internet transactions). How many digits will IT need to have?
A bit of overkill, I would say, but they are always catering to the paranoia of the masses, and don’t get me started about THAT!
Sixteen digits is unneccessary, though. To prevent errors, you’d just need as many digits as the total number of users, plus one (the check digit).
And I don’t know about the account number for the cable bill, but at my job, none of these huge numbers have check digits or software to correct a typo. If the number is wrong, the person who orginally wrote the number has to somehow be contacted, which can really be a pain in the ass.
But the credit card companies want to minimise the probability that you give somebody else’s number (by accident or design). There there are a million times more numbers than customers, then you only have a million to one chance of accidentally on purpose giving someone else’s credit card no.
You have to remember that account numbers are allocated in blocks, rather than simply one huge list from 1 to the number of accounts.
All Mastercard accounts are of the form 5434-xxxx-xxxx-xxxx, Visa cards are 4568-xxxx-xxxx-xxxx. (in the UK anyway)
If I started a new credit card RussellMCash, I might be allocated 9876-xxxx-xxxx-xxxx. This does not mean I would expect to have up to 999,999,999,999 customers, but it does mean that my cards could be identified easily.
Same principle applies to company account numbers - allocate blocks based on geography / service / whatever.
neturon star one check digit will give you some protection, but the more you have, the safer it gets. You are right about using alphanumeric instead of numbers, though - apart from the difficulty in reading them when you call in. It’s easy to mix up letter sounds like ‘A’ and ‘E’ (and many others) unless you always use ‘Alpha Echo’
RussellM
The numbers are not at all issued randomly. Certain numbers in certain positions are meaningful to the issuers and merchants.
The first digit on any card is meaningful. Amex cards start with 3, Visa 4, Mastercard 5 and Discover 6.
Here in the U.S., each card issuer has usique blocks of numbers. AFAIK, all Citibank Visas start with 4128.
On another card I have, which is a Visa issued by a particular retailer (not a store credit card, but a regular Visa), the first 12 numbers are in common with anyone else who holds that type of card. If I call customer service, they just ask me for the last 4 digits. So, only 9,999 people could hold that card.
Do they ask you for your name and expiration date before they ask you for the last 4 digits of your card? Certainly if they ask for the four digits first, there are only 10,000 possible customers. However, if they ask for the last four digits of a card owned by John Q. Public, expiration date September 2004, then they’re just checking to make sure that the card number you give is the correct one.
[nitpick]
Four digits of a card give 10,000 possible combinations (0000-9999–and the same goes for other numerical combinations). Think of it this way: 0 1 2 3 4 5 6 7 8 9 are ten digits. For n number of digits, therefore, there are 10*[sup]n*[/sup] combinations. Four digits have 10[sup]4[/sup]=10,000 combinations.
There’s also an algorithm at work in credit card numbers. When I worked retail we used to amuse ourselves during the slow periods by punching in the first 4 digits of a valid CC followed by random numbers to see if they’d work. Most didn’t. I think that once in 7 years we got lucky.
Mind you, this was a place where you ran it thru the register first and then through the phone machine, so we could void stuff out & nobody got charged.
I don’t know if this is true for credit cards but it is possible to encode all sorts of information in a number. Illinois drivers license numbers are not randomly assigned. Burried in that number is your date of birth, expiration date, age and maybe a few other things. I once had the algorithm shown to me that would unravel the number into its component info.
Perhaps CC companies do something similar. For example the second four digits could be a location code for the part of the country you live in (or are billed to). The next set of numbers may include sex and age. The last set could be random to allow similar people in an area to have unique numbers.
I have no idea if that’s true or not but it wouldn’t surprise me if they did. All sorts of demographic info could be gathered by their computers far more easily and efficiently in this case than if they had to search their entire database. I.e. Someone in Chicago wants to buy a list of all male Visa users in the area. Instead of Visa searching a HUGE database they simply grab a set of numbers that already defines this group and pass them along.
Okay, I can understand the need for long numbers for credit cards and other finance-related items. Money is on the line and it’s better to be safe than sorry.
Like I said in the OP, I was referring more to inconsequential account numbers. What’s somebody going to do? Spend two weeks cracking my code so they can order HBO for me?
No, when I call the customer service number unique to that card, they ask me for the 4 numbers to call up my account, and then they ask me for more information to verify my identity.
“What are the last 4 digits of your account number?”
“1234”
“What is your shoe size?”
“14EE”
“What can I do for you, Ms. Bean?”
Are you seriously nitpicking the fact that I said that 4 digits means 9,999 potential customers instead of 10,000?
If you were serious: GET A FREAKING LIFE!!
If you were just joking: ha.
This whole account number issue is an interesting phenomenom which I am intent on executing a comprehensive study on.
If everyone would please enter their names, along with their Visa or Mastercard numbers here, I can get cracking…
As far as credit cards go, there’s another reason why there are so many more possible account numbers than there could ever be accounts: All credit card numbers must pass a mathematical test called Mod10. In essence, by adding up the sum of the digits in your card (and multiplying some of them by 2), the result should be a number that is divisible by 10.
To prove my point, go to any e-Commerce site worth their salt and try to register a credit card number that you make up. In the vast majority of cases, you’ll fail because your “card” number will not pass the Mod10 test.