Helping Cashiers Catch Their Own Mistakes

There are lots of things you can do to help prevent errors, but it's impossible to ensure that users are always successful. In fact, due to the law of diminishing returns, it's not worth it to try to prevent (or catch) every possible error. Instead, after we've folded in the error-preventing and error-catching features appropriate to a particular client, we then look for opportunities to make mistakes apparent so that cashiers can catch them on their own.

In general, we strive (a) to make it evident what the system "believes" to be true and (b) to present that information in ways that easily allow verification. Here's a simplistic example: In a retail situation, errors might happen when an item is on sale, but the system doesn't correctly mark it down. Errors like these are virtually impossible to prevent, since they depend on human beings for information. Thus, we need to also rely on human beings to catch the error when it happens.

Most cashiering interfaces show a running "ticket" that confirms what the system believes to be on the order. In our "sale" example, we'd naturally want to make sure that the ticket prominently displayed the name and price for the item. But a highly usable Point of Sale would go further. The optimal solution would depend on retailer, but let's say that sale items were marked in a special way on the screen. That way, the cashier could more easily determine whether the system believed that the just-scanned item was on sale. And it would become even more powerful if the style of that on-screen marking were to match the physical "on sale" price tag attached to the item.

Another way we can help cashiers catch errors is to provide redundant displays where appropriate. At Wendy's, for instance, I designed the interface with condiment buttons that stick down when selected. This allows default condiments for a particular sandwich to be pre-selected when the sandwich is ordered, making it easy to see what the system believes should come on a particular sandwich. It also allows cashier to verify any adjustments to the default build without constantly having to look at the ticket. In this way, we provide a redundant method for verification, and we reduce the need for cashiers to constantly shift their attention from the point of interaction to the ticket and back again. And that helps speed things up too!

Note: The sticky condiment button interaction has been copied by several vendors for use with numerous other clients.