Computers in working order and with correct software don’t make mistakes. People, however, make plenty of mistakes (including writing bad software or breaking computers). In quality circles, there’s a Japanese term, poka yoke, which roughly means ‘error avoidance’. The idea is to avoid errors by making them too obvious for them to occur. For example, consider a SIM card in your phone. The little diagonal corner means it only goes in one way. If you put it in the wrong way, it is obviously wrong.
To be successful at poka yoke, you have to be able to imagine what a user might do wrong and then come up with some way to make it obvious that it is wrong. There are examples of this all around us and we sometimes don’t even know it. For example, what do your credit card number, your car’s VIN code, and a UPC code on a can of beans have in common?
The answer is that they all are long strings of digits which are notoriously difficult for humans to enter correctly. People miss numbers or transpose them. So people who write applications that take numbers like this often want to check to make sure the person didn’t make a mistake.
Of course, a number is a number, right? If I tell you to enter a five digit zip code, I can figure out if you put in four digits or six, but it is hard to know if 77508 or 77580 is the one you meant. That’s why long and important numbers have one or more check digits.
A check digit is like a checksum or a CRC — you comp ..
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