User interface design for vending machines – answer to puzzle

Raymond Chen

Last time, we ended a discussion of vending machine design with a short puzzle: What problems do you see with numbering the products from 1 to 99?

I’m not saying that these are the only possible answers, but they are ones that came to mind when I thought about it.

  • Product codes less than 10 would be ambiguous. Is a “3” a request for product number 3, or is the user just being slow at entering “32”? Solving this by adding a leading zero will not work because people are in the habit of ignoring leading zeros.
  • Product codes should not coincide with product prices. If there is a bag of cookies that costs 75 cents, users are likely to type “75” when they want the cookies, even though the product code for the cookies is 23.

Ilya Birman was the first to point out the “bounce-effect” problem, thereby ruling out product codes like 11, 22, and 33.